Beta: use MemcachedPhpBagOStuff if running under HHVM

When running under HHVM, the Beta app servers should use MemcachedPhpBagOStuff.
When using PHP 5.3, they should use MemcachedPeclBagOStuff.

Change-Id: I50b342ffe59f5c1c51670d541a4e25616365d13b