False 1.0 similarity for PNG on Mac OS El Captain

[Nakilon]

I was using pHash via Ruby gem phash on a bit outdated Mac OS. After installing El Captain on new HDD (and so reinstalling pHash) it started to give false positive (1.0) similarity when at least one image is PNG. For example:

• http://static1.squarespace.com/static/546d6314e4b0dce5377a07f3/54f7ad0ce4b0aea85187fcc9/54f7ad34e4b0aea851882685/1425518500367/
• http://static1.squarespace.com/static/546d6314e4b0dce5377a07f3/54f7ad0ce4b0aea85187fcc9/54f7ad3ae4b0aea851882c90/1425518468940/

Values for JPEG didn't start to be wrong so when I convert the above images to JPEG, it says 0.46875
I forced an old version of gem ffi, but it had no effect -- probably smth changed in these binaries:

==> Installing dependencies for homebrew/boneyard/phash: cimg, x264, lame, xvid, ffmpeg, flac, libogg, l
==> Installing homebrew/boneyard/phash dependency: cimg
==> Downloading https://homebrew.bintray.com/bottles/cimg-1.6.9.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring cimg-1.6.9.el_capitan.bottle.tar.gz
🍺  /usr/local/Cellar/cimg/1.6.9: 72 files, 14.3M
==> Installing homebrew/boneyard/phash dependency: x264
==> Downloading https://homebrew.bintray.com/bottles/x264-r2668.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring x264-r2668.el_capitan.bottle.tar.gz
🍺  /usr/local/Cellar/x264/r2668: 10 files, 3.3M
==> Installing homebrew/boneyard/phash dependency: lame
==> Downloading https://homebrew.bintray.com/bottles/lame-3.99.5.el_capitan.bottle.1.tar.gz
######################################################################## 100,0%
==> Pouring lame-3.99.5.el_capitan.bottle.1.tar.gz
🍺  /usr/local/Cellar/lame/3.99.5: 26 files, 2M
==> Installing homebrew/boneyard/phash dependency: xvid
==> Downloading https://homebrew.bintray.com/bottles/xvid-1.3.4.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring xvid-1.3.4.el_capitan.bottle.tar.gz
🍺  /usr/local/Cellar/xvid/1.3.4: 9 files, 1.3M
==> Installing homebrew/boneyard/phash dependency: ffmpeg
==> Downloading https://homebrew.bintray.com/bottles/ffmpeg-3.1.1_1.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring ffmpeg-3.1.1_1.el_capitan.bottle.tar.gz
==> Caveats
The native FFmpeg AAC encoder has been stable since FFmpeg 3.0. If you
were using libvo-aacenc or libaacplus, both of which have been dropped in
FFmpeg 3.0, please consider switching to the native encoder (-c:a aac),
fdk-aac (-c:a libfdk_aac, ffmpeg needs to be installed with the
--with-fdk-aac option), or faac (-c:a libfaac, ffmpeg needs to be
installed with the --with-faac option).

See the announcement
https://ffmpeg.org/index.html#removing_external_aac_encoders for details,
and https://trac.ffmpeg.org/wiki/Encode/AAC on best practices of encoding
AAC with FFmpeg.
==> Summary
🍺  /usr/local/Cellar/ffmpeg/3.1.1_1: 236 files, 50.1M
==> Installing homebrew/boneyard/phash dependency: flac
==> Downloading https://homebrew.bintray.com/bottles/flac-1.3.1.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring flac-1.3.1.el_capitan.bottle.tar.gz
🍺  /usr/local/Cellar/flac/1.3.1: 52 files, 2.2M
==> Installing homebrew/boneyard/phash dependency: libogg
==> Downloading https://homebrew.bintray.com/bottles/libogg-1.3.2.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring libogg-1.3.2.el_capitan.bottle.tar.gz
🍺  /usr/local/Cellar/libogg/1.3.2: 96 files, 457.3K
==> Installing homebrew/boneyard/phash dependency: libvorbis
==> Downloading https://homebrew.bintray.com/bottles/libvorbis-1.3.5.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring libvorbis-1.3.5.el_capitan.bottle.tar.gz
🍺  /usr/local/Cellar/libvorbis/1.3.5: 158 files, 2.3M
==> Installing homebrew/boneyard/phash dependency: libsndfile
==> Downloading https://homebrew.bintray.com/bottles/libsndfile-1.0.26.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring libsndfile-1.0.26.el_capitan.bottle.tar.gz
🍺  /usr/local/Cellar/libsndfile/1.0.26: 48 files, 1.8M
==> Installing homebrew/boneyard/phash dependency: libsamplerate
==> Downloading https://homebrew.bintray.com/bottles/libsamplerate-0.1.8.el_capitan.bottle.2.tar.gz
######################################################################## 100,0%
==> Pouring libsamplerate-0.1.8.el_capitan.bottle.2.tar.gz
==> Caveats
Unless this formula is built with libsndfile, the example program,
"sndfile-resample", is broken and hence, removed from installation.
==> Summary
🍺  /usr/local/Cellar/libsamplerate/0.1.8: 26 files, 3.0M
==> Installing homebrew/boneyard/phash dependency: mpg123
==> Downloading https://homebrew.bintray.com/bottles/mpg123-1.23.6.el_capitan.bottle.tar.gz
######################################################################## 100,0%
==> Pouring mpg123-1.23.6.el_capitan.bottle.tar.gz
🍺  /usr/local/Cellar/mpg123/1.23.6: 22 files, 687.9K

See some background about installation: toy/pHash#8

[Nakilon]

I assume the difference is in cimg library. The changes of it to brew since I've installed phash previously are these two commits:
Homebrew/brew@10a3aa2
Homebrew/brew@250fcc1
so it was upgraded from 1.6.3 to 1.6.9