Drop 0.5 support

This release is backwards source compatible with earlier versions.
However, with the release of julia 0.6, no further testing on 0.5 will
be performed. As a result, this release preemptively drops 0.5 support
and patch releases in this series may take advantage of 0.6-only features
and syntax in preparation of julia 0.7/1.0.

As of this release, Cxx.jl has support for precompilation of Cxx itself
(precompilation of dependent packages is not supported), significantly
reducing package load times.