Fix docblock
Use `HHVM_VERSION` instead of `HPHP_VERSION`.

Seems there's a chance that `HPHP_VERSION` may be removed one day (see: facebook/hhvm@1632ca1).
Merge branch '3.8'
Use `HHVM_VERSION` instead of `HPHP_VERSION`.

Seems there's a chance that `HPHP_VERSION` may be removed one day (see: facebook/hhvm@1632ca1).