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

Follows-up 67737a2.

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 <travis-ci/travis-ci#7368>.

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

Bug: T75175
Change-Id: Ic59936734211b68d9701119a551d3bd1b83c845e

Author: Krinkle

.travis.yml

@@ -7,6 +7,12 @@
 # complement that setup by testing MediaWiki on travis
 #
 language: php
+# Using HHVM-3.6+ requires Trusty (Travis default: precise)
+# https://docs.travis-ci.com/user/languages/php#HHVM-versions
+# https://github.com/travis-ci/travis-ci/issues/7368
+sudo: required
+group: edge
+dist: trusty
 
 matrix:
   fast_finish: true
@@ -44,7 +50,7 @@ before_script:
       --pass travis
       --dbtype "$dbtype"
       --dbname traviswiki
-      --dbuser travis
+      --dbuser root
       --dbpass ""
       --scriptpath "/w"