build: Change Travis image from precise to trusty (Fix HHVM build)

build: Change Travis image from precise to trusty (Fix HHVM build)

Follows-up 67737a267f.

Per <https://docs.travis-ci.com/user/languages/php#HHVM-versions-on-Trusty>
using hhvm-3.12 requires a trusty image. The upstream source at
<http://dl.hhvm.com/ubuntu/dists/> doesn't provide it for precise.

In addition to changing to trusty, we also need to:
* Set 'sudo: required' and 'group: edge'.
  Because lightweight "sudo: false" containers are still
  precise-only. There is a beta test and I tried it, but it
  doesn't support out apt-get install commands yet.

* Change mysql user from 'travis' to 'root'.
  The fact that 'travis' supports creating databases on precise
  is undocumented. One is supposed to use 'root' for this,
  which is required on Trusty.
  <https://docs.travis-ci.com/user/database-setup/#MySQL>
  Without this, install.php fails.

More details at <https://github.com/travis-ci/travis-ci/issues/7368>.

This config was tested and passed at
<https://travis-ci.org/Krinkle/mediawiki/jobs/204463664>.

Bug: T75175
Change-Id: Ic59936734211b68d9701119a551d3bd1b83c845e