Add test targets for all Ember channels.

Now you can run:

* `make test_all` to run all test targets.
* `make test_channels` to run the tests against all three Ember
 channels.

I have also updated the Travis config to use `make test_all`.

I took a look at using multiple Travis builds, but at this point the test suite is actually faster run all together than it would be as separate builds. This is due to the duplication of running `npm install` and downloading the assets again for each build.