Enable builds for PHP 5.6 and HHVM

Seems that the HHVM team has recently [finished implementing the Imagick extension](https://github.com/facebook/hhvm/commit/9ce72f087fafb6359d6794a92aa4d9f54d68ba24).

I've added builds for both HHVM and PHP 5.6 while allowing these to fail without affecting the build status. This enables us to monitor and find any issues with these versions without continually running tests against them locally.