hhvm: provide hhvm-api-$VERSION

As hhvm does not guarantee ABI compatibility, all extensions should be
rebuilt once we have changed the ABI version - but we don't need them to
be rebuilt for every version of this package, so we:

* Expose the API version via hhvm --version
* Change it according to our ABI stability tests
* Add to the debian control file a Provides: hhvm-api-$VERSION

This will allow a saner control of dependencies in HHVM extensions, as
we already do with PHP

Change-Id: Iad305e0df95bbc6667752ccd9f97b57b2708d583
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>