Enable multi-channel testing.

Adds the following tasks:

* `test:local` (alias to previous `test` task)
* `test:release` (tests against current release builds of Ember)
* `test:beta` (tests against current beta builds of Ember)
* `test:canary` (tests against current canary builds of Ember)
* `test:all` (tests against all channels of Ember)

Changes the default Travis test script to `test:all`.