build: Update Travis CI config to use fast Trusty containers

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