Enable multi-version testing

`test/index.html` changes:

* Adds ability to test against multiple versions of Ember via the
  `emberchannel` query param (defaults to local Bower version).
* Adds ability to test against multiple jQuery versions via the `jquery`
  query param (defaults to local Bower version).

Adds the following tasks to `Gruntfile.js`:

* `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`.