Introduce HHVM's cookbook.

The package recipe installs HHVM from the provided package repository.
The default recipe just refers to the package one.
Make HHVM available for phpenv.
Composer's config file needs to be created once only.
Install composer for HHVM.
Install phpunit for HHVM.
Install composer for HHVM only when necessary.
HHVM's version is now explicit.
Add a metadata.rb in HHVM's cookbook.
Merge pull request #245 from travis-ci/hhvm

Support for HHVM within the PHP VM