re-enabled query cache of ringo-sqlstore

see https://groups.google.com/d/msg/ringojs/Ji7BFWyODYQ/s_mb9hIY9pcJ resp. https://github.com/TechEmpower/FrameworkBenchmarks/issues/289#issuecomment-20669224 for an explanation of what the query cache does.
Merge pull request #390 from grob/master

re-enable query cache of ringo-sqlstore