travis: Try hhvm, php 5.4, php 5.5, php 5.6

* `hhvm-nightly` seems broken at the moment (installer fails)
  https://travis-ci.org/wikimedia/mediawiki-core/jobs/37144078
* Contrary to what ce51ca848dc5eb59fe2b9c1a9422a3e63c43b051 claims, adding more versions doesn't delay the build. And the information is just as easy to consume on pages like https://travis-ci.org/wikimedia/mediawiki-core/builds/37144077 no matter whether there's 2 versions or 6 versions.