Update Travis CI build configuration

* Target 'hhvm-nightly', not 'hhvm'.
* Don't target Zend PHP versions other than 5.3.
  There is value in targetting different versions of the PHP interpreter, but
  in the short term there is more benefit to be had from targetting just HHVM
  and PHP 5.3 (results are produced more quickly and are easier to interpret).
* Don't update submodules manually; Travis does that by default if there is a
  .gitmodules file in the repository root.
  <http://docs.travis-ci.com/user/build-configuration/#Git-Submodules>
* Update Composer before installing PHPUnit.
* Make the wiki name and dbname agree.
* Allow Composer to install whatever is specified in composer.json, rather than
  hard-coding an install of PHPUnit 3.7 (3.7 is incompatible with HHVM).
* Ping freenode#mediawiki-core with the test results, but only when the build
  status changes. We can turn it off if it gets annoying.
* Don't hack include_path or date.timezone ini settings; those were kludges
  needed to get HHVM to run at all at, but they're no longer needed.
* Invoke hhvm via 'php'; Travis ensures that it points to the target
  interpreter.

Verified on my personal fork:
 <https://travis-ci.org/atdt/mediawiki-core/builds/27983653>

Change-Id: I442b40e0d3e31d3cbd963e1219578842f4564004