Rename wfIsHipHop() to wfIsHHVM()

The former is only really used to refer to the older hphp compiler.
Now all docs pretty much exclusively are referring to HHVM and
that's the project's name going forward.

While we're here, swap HPHP_VERSION for HHVM_VERSION. Either work,
but the latter is more correct.

Only affected extension (in Git or SVN) is GWToolset, fixing in
I3a490ef.

Change-Id: I81e4ce5d14e344e67045050261f91b3c0159f222