Enable HHVM debugging REPL

* Enable the HHVM debugging server.
* Load wm-core's index.php when attaching to the debugging server.

Use by logging in to your vm and running:

    hhvm --mode debug --debug-host ::1

Or use from the host computer with:

    vagrant ssh -c 'hhvm --mode debug --debug-host ::1'

Once connected, type '?' for a list of basic commands. For example, `g`
will list current globals or `=$IP` will show the current value of the
$IP variable.

Bug: 67756
Change-Id: I93aa65d4be1fbcd1285fc0ad715e4d9a6d9d3226
Merge "Enable HHVM debugging REPL"