Ember beta or Ember Data beta don’t get properly installed after e12c4ca

Hey, this is extracted from #207, which I thought was the same problem but [isn’t](https://github.com/ember-cli/ember-try/issues/207#issuecomment-448882870)!

As @kategengler suggested, when I [tried](https://github.com/ember-cli/ember-try/issues/207#issuecomment-449055834) installing e12c4ca, it worked as expected with the `travis-web` [`ember-try` configuration](https://github.com/travis-ci/travis-web/blob/889fb9db2e1ed5b7e48c3c4e20fafae3ece207df/config/ember-try.js) of `'ember-source': 'beta'` and `'ember-data': 'beta'`. After that commit, `ember try:one ember-data` fails because the `ember-source` package isn’t installed.

You can see an example of such a failure [here](https://travis-ci.org/travis-ci/travis-web/jobs/433639308#L606-L609).

Let me know if there’s any other information I can provide. Thanks for all the work on this, I appreciate the automation so we can try to find issues with the betas early!