Adding PHP 5.6 to build matrix.
Enable fast finish.
Setup experimental HHVM in CI build matrix; allow fails.

Excluding non available extensions from HHVM CI build.
Do not build/pecl install extensions bundled with HHVM.

Removing php -i debugging in before_script as it causes
the build with HHVM to hang.
Fix type for $options in `Cache` docblocks.
Adding description for `$name` param in `Cache` docblocks.
Refactoring; use `$keys` intead of `$params['keys']` when iterating.
Refactoring; Use `try..catch` instead of checking config array in `Cache`.