Add a conflict with the lang/gcc6-devel port as we've had it in lang/gcc5
(for lang/gcc5-devel) for a fair while. This mirrors what we did for the
lang/gcc5-devel port earlier this week.

Also make Python a build-only dependency when building Java
(via JAVA_USES=python:build) which makes the installed port/package
quite a bit more lightweight.

Bump PORTREVISION for those two changes.