Added HHVM build to Travis CI configuration.

Includes a script that updates HHVM to the latest nightly build as the current
release is missing some things that Phake requires (for example, the `Reflection`)
class.
Bare minimum to stop HHVM build from producing fatal errors.
Echo real HHVM version after the nightly build is installed.
Merge pull request #133 from jmalloc/hhvm

Add HHVM to Travis config.

Thanks for the contribution!