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).

This project depends on oraclejdk8. This commit explicitly defines 'trusty' to be used, which should make oraclejdk8 available again.
igniterealtime/ofmeet-openfire-plugin,igniterealtime/ofmeet-openfire-plugin,igniterealtime/ofmeet-openfire-plugin