Cache modules for jsx grunt tasks in react-tools/.module-cache

As of Commoner v0.6.11, the default is to put the cache files in `outputDir/.module-cache`, which used to be `build/modules/.module-cache` before this commit. That still happens when you run `bin/jsx` directly, just not for grunt tasks anymore.

The module cache needs to be cleared much less often than `build/modules`, so it doesn't make sense to throw away all that work.