build: Migrate to Trusty image to fix HHVM
build: Migrate to Trusty image to fix HHVM

Since the last build (4 months ago) HHVM dropped support for Precise,
at which point Travis also removed support for php:hhvm on Precise.

Use the "new" Trusty images instead.

Change-Id: I5d76a229a57a35a1d9bd69b309d51041e054d327

wikimedia/RelPath,wikimedia/RelPath