Build against and require Java 1.8

As discussed on PR #319 for GUACAMOLE-220, and as discussed on the dev@ mailing list, continuing to support Java 1.6 at all levels of guacamole-client makes less and less sense:

• Java 1.6 reached end-of-life some time ago.
• Java 1.7 has ceased receiving further public updates.
• Other mainstream projects which rely on Java have already moved on to 1.7+.

Though guacamole-common should likely remain as compatible as possible, moving all other Java components forward to a 1.8 build seems best, with the upcoming major 1.0.0 release being an ideal time.

The only non-EOL Linux distribution I've been able to find so far that lacks Java 1.8 is Ubuntu 14.04 LTS, which is reaching EOL in April 2019.