Changed TravisCI to explicitly use Trusty images.
Changed TravisCI to explicitly use Trusty images.

Xenial is not yet supported and aparently recent changes make
xenial revert to precise rather than trusty (latest supported).

This may or may not fix the build issue.

AeonGames/AeonEngine,AeonGames/AeonEngine,AeonGames/AeonEngine,AeonGames/AeonEngine,AeonGames/AeonEngine