Indicate the actual version of HHVM in use

Strings like "5.6.99-hhvm" are not version numbers but merely
a way for HHVM to pass version checks. They should not be
displayed in the UI.

This affects Special:Version, the API (action=query&meta=siteinfo),
the installer welcome page, and the debug toolbar.

Follows-up d09ab9001ffe.

Change-Id: Ia99dca64779e9c4eaddf5f0e0101674d029b8d55
Merge "Indicate the actual version of HHVM in use"