Travis: Add HHVM as a PHP runtime
Memcache: Default to empty array in getMultiple

HHVM appears to return false if no keys were found.
Memcache: Fix incr/decr inconsistencies with HHVM
Memcache: Fix max key length

Turns out it's 250 rather than 255. HHVM returned false on keys longer
than the max while other runtimes did not. This didn't result in any
failures because the hash at the end still didn't conflict although it
was possible.
Merge branch 'travis-hhvm'