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
output/.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.
Merge pull request #72 from benjamn/module-cache

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