Add HHVM to Travis

Currently 1 test is still failing, so it's set up to allow for failures.
See: https://github.com/facebook/hhvm/issues/2018

Signed-off-by: Jeff Chan <jefftchan@gmail.com>
Only install runkit / pear selenium when normal PHP

Also, use Travis' composer binary

Signed-off-by: Jeff Chan <jefftchan@gmail.com>
Merge pull request #1056 from jeffchan/add-hhvm-travis

Add HHVM to Travis