Use the native implementation for Memcached::decrement

Summary: Calling Memcached::decrementByKey with an empty string causes all
decrement operations to hash to a single node in the server list. I suspect
the intention was to pass along a NULL, but calling the native implementation
like ::increment seems cleaner either way.
Closes https://github.com/facebook/hhvm/pull/4307

Reviewed By: @fredemmott

Differential Revision: D1709908

Signature: t1:1709908:1417809973:2c362a52f644b48cf5f141fea8317b1c3f63792c