Travis: explicitly define distribution
Travis: explicitly define distribution

Recently, the default distribution used by Travis was changed from 'trusty' to 'xenial'. See https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment

In Xenial, oraclejdk8 is not available (although openjdk8 is).

Openfire depends on oraclejdk8. This commit explicitly defines 'trusty' to be used, which should make oraclejdk8 available again.

GregDThomas/Openfire,igniterealtime/Openfire,guusdk/Openfire,akrherz/Openfire,akrherz/Openfire,guusdk/Openfire,guusdk/Openfire,GregDThomas/Openfire,igniterealtime/Openfire,igniterealtime/Openfire,speedy01/Openfire,GregDThomas/Openfire,speedy01/Openfire,guusdk/Openfire,GregDThomas/Openfire,GregDThomas/Openfire,speedy01/Openfire,akrherz/Openfire,akrherz/Openfire,speedy01/Openfire,igniterealtime/Openfire,guusdk/Openfire,akrherz/Openfire,speedy01/Openfire,igniterealtime/Openfire