Selectively turn on enableTestCommands automatically.

This is another possible option for resolving #152 that doesn't rely on having another set of configuration files.

I've tested this with various versions of mongod and mongos from 2.0 - 2.8, and it works nicely on all of them.
This will just extract the numeric part of the version. Not sure we need to be finer grained than that at this point.