Add Travis CI job configuration for testing MediaWiki on HHVM

Add Travis CI job configuration for testing MediaWiki on HHVM

Travis CI is a popular continuous integration service that provides free
automated testing to FOSS projects. Ten days ago, the HHVM team announced that
Travis CI's build environment provides HHVM by default. This patch contains a
Travis CI job configuration that runs MediaWiki's test suite using HHVM.
Because we replicate changes to GitHub, merging this change and enabling the
Travis CI hook in the repository configuration on GitHub will cause Travis CI
to run the tests on each commit.

Change-Id: Ia18cba5facb7f0b506e962df2f114338af571b8a