Update and lock broccoli dependencies

This will help prevent unintended breaking dependency updates.

- Updates broccoli dependency to v0.7.2
- Updates broccoli-es6-concatenator to v0.1.4
- Updates broccoli-static-compiler v0.1.4
- Locks broccoli-replace version to v0.1.5
- Locks ember-cli to v0.0.21

I wasn't sure whether or not to include the changes in https://github.com/stefanpenner/ember-cli/pull/267