Fixing HHVM compatibility of the proxy generator when generating proxy classes with methods using `callable`

Also splitting logic into smaller, simpler methods
Moving parameter type detection to a separate method
Enabling HHVM-nightly and PHP 5.6 in builds, disallowing hhvm failures
Avoiding duplicate calls to `getParameterNames`
Tests pass for PHP 5.6: not allowed to fail anymore.
Merge pull request #330 from doctrine/hotfix/hhvm-proxy-generator-compat-with-callable-type-hints

HHVMm proxy generator compatibility with callable type hints