Fixes the reference to callback in the method ensureIndex.

Message:
     ReferenceError: callback is not defined
   Stacktrace:
     ReferenceError: callback is not defined
    at MonckooseCollection.ensureIndex (/Users/nukulb/src/hubba-node/node_modules/monckoose/lib/monckoose.js:133:3)
    at MockCollection.(anonymous function) [as ensureIndex]