7.1.0: Drop PHP 5.5

• Dropped official support for PHP 5.5 - code will still work but is no
  longer tested on Travis CI. This keeps XP6 compatibility.
  (@thekid)

7.0.0: XP7 compatibility

• Adopted semantic versioning. See xp-framework/rfc#300 - @thekid
• Added version compatibility with XP 7 - @thekid

6.5.6: Extract from XP Core

• Heads up: Split library from xp-framework/core as per xp-framework/rfc#298
  (@thekid)