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
Increase memory_limit for PHP from 512M to 1G.
Add Go 1.2
Update changes.md
Fixes metadata for the PHP cookbook: include hhvm dependency.

Related to #245.
Merge pull request #244 from dmalikov/master

multi-ghc approach without ghc-select script and symlinking
ghc_find in separate executable