Prevent test/demo app config from leaking into consuming app.

As of Ember CLI 0.0.45 and up, the file located at
`config/environment.js` will be used to return the addon's own
configuration.  That conflicts with the way this library is using
`config/environment.js` (for the testing/demo app).

This simply overrides the default config hook to return nothing.
Merge pull request #75 from rwjblue/do-not-leak-config

Prevent test/demo app config from leaking into consuming app.