Test against all Ember channels.

* Update `ember-dev` to 56b02fa which allows testing against each of the
  channels via a URL param.
* Add additional suite (`all`) to run against each channel.
* Add `.travis.yml` to override default test script (so we can test against
  each channel in CI only).
Merge pull request #57 from rjackson/test_against_multiple_versions

Test against all Ember channels.