specifying trusty/edge distribution to get newer hhvm
specifying trusty/edge distribution to get newer hhvm

See:
- https://github.com/travis-ci/travis-ci/issues/5328
- https://docs.travis-ci.com/user/languages/php#HHVM-versions
- https://docs.travis-ci.com/user/trusty-ci-environment#PHP
honeybee/honeybee