Documenting Helper::_attributes().
Documenting alternative syntax and unused param in Helper::_attributes().
Fix typo which prevented opcache from being enabled in CI.

Refs #1069.
Document and typehint in depends.php.
Adding php -i in CI build for debugging.
Use beta version of zendopcache for PHP < 5.5.

Fixes #1069.
Adding HHVM to CI build matrix; allow fails.

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