HHVM on beta: use memc04 & memc05 for ObjectCache

I have a suspicion that some of the exceptions we're seeing with HHVM on Beta
have to do with the fact that two instances of the same wiki ($wgDBname,
database, etc.) share the same memcached servers, even though they load
different extensions. So let's have each interpreter use its own memcached
mini-cluster.

Change-Id: I6ed157073a9850c603fe39a557ff1041ddf463f4