HHVM Fixes

Mockery 0.9 or higher is required for hhvm. As of January this year, the master branch was aliased as 0.9, so we are technically still loading the same version, just in a cleaner way.

PHPUnit 4.0 is required for proper hhvm support. PHPUnit 4.0 will be tagged as stable on 7th March this year.

HHVM doesn't support xdebug, so I have marked that test as skipped. So we can see skipped test warnings, I have added the verbose flag to phpunit when we run it in travis.yml.