[BUGFIX beta] Update to latest Backburner.js.

From https://github.com/ebryn/backburner.js/pull/54:

    Fix a bug where debounced item in index 0 isn't
    removed unless debounce for that target/method is called again, but
    then it's in index 0 and then isn't removed

[Compare
view](https://github.com/ebryn/backburner.js/compare/d6bc31f978eb0ef0c8288c1bef9d08420caff00a...afa5f6e22541772fbc702e2e1d7a3afc480b66b1)
Merge pull request #3887 from rjackson/update_ember-dev

[BUGFIX beta] Ensure that each branch is bundled before building.
[DOC beta] Clarify context of a component's block.
[BUGFIX beta] Add preventDefault option to link-to and action.

Resolves #3737
Merge pull request #3886 from rjackson/update_backburner

[BUGFIX beta] Update to latest Backburner.js.
Merge pull request #3855 from rjackson/add_prevent_default_option

[BUGFIX beta] Add preventDefault option to link-to and action.
Merge pull request #3862 from rjackson/add_action_context_tests_for_component

[DOC beta] Clarify context of a component's block.
Ensure 1.3.0 features are on by default in master/canary.

The 1.3.0
[features.json](https://github.com/emberjs/ember.js/blob/v1.3.0-beta.2/features.json)
for reference.
Merge pull request #3890 from rjackson/ensure_beta_features_are_on_in_master

Ensure 1.3.0 features are on by default in master/canary.
[BUGFIX beta] Handle beta branch builds properly.

The initial implementation of the `FORCE_BRANCH` flag did not
properly account for all scenarios.  This attempts to fix an
issue with builds of commits pushed directly to the beta/stable
branches.