build: Update Travis CI config to use fast Trusty containers
We tried this before when Trusty containers were still in Beta,
but the issues we ran into at the time have since been fixed.
1. Missing package djvulibre-bin: Fixed.
https://github.com/travis-ci/apt-package-whitelist/issues/4036
2. MySQL launch problems: Fixed
https://github.com/travis-ci/travis-ci/issues/5837
https://github.com/travis-ci/travis-ci/issues/6842
Also add HHVM 3.18 to the test matrix (latest currently preinstalled
version of HHVM).
Change-Id: I2f56e303c01f069207fb086e784e49292ea4eb58