Add HHVM to Travis config.

Similar to #132 this PR adds a HHVM build to the Travis CI configuration. It also has a build step that updates HHVM to the latest nightly build, as the current HHVM release is missing PHP features that Phake requires.

I've also done just enough so that the tests do not produce fatal errors, though a good handful still fail.
While those failures are most certainly due to differences between HHVM and PHP proper, I hope you'll consider supporting HHVM even if it did mean some changes to the Phake codebase in the meantime :)