Allow testing against multiple versions of Ember & jQuery.

* 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).
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`.
Merge pull request #1709 from rjackson/enable-multi-version-testing

Enable multi-version testing