Changed makeModules to return a broccoli MergedTree

I was bashing my head off the desk trying to figure out why my Brocfile wasn't compiling. After some digging I realized that broccoli-dist-es6-module was returning an array of trees, not a tree. The fix was simple enough, turn the result into a merged tree. I feel like broccoli-dist-es6-module should do this itself to maintain a consistent experience for Broccoli users.

Let me know what you think. Also thanks a ton for all your work around this, it really helped me get up to speed a lot faster than I would have otherwise. I was a globals scumbag before ES6 tooling.