Drop julia 0.5 support.

This release drops the compat dependency and julia 0.5 syntax and compatibility, in favour of 0.6.
We aim for the BioJulia-verse of packages to be written in modern, clear, idiomatic julia, and as the language matures and approaches version 1.0, maintaining compatibility with previous releases of julia with significant syntax differences detracts from this goal. So with BioSequences v0.7, we have dropped support for julia v0.5.