[BUGFIX release] Fix #3681.

`Em.arrayComputed` is not presently compatible with lazy CPs. There are a few
issues.

1.  To keep refs up to date, observers stay around as long as the object does.
2.  Those observers can repopulate the cache, but when they do so they fail to
    add dependent keys when the cache was empty.

This commit works around 2. by not removing them in the first place.
Essentially the semantics for `ArrayComputedProperty`s differ from standard
`ComputedProperty`s with regards to laziness.

These observers need to be managed better.  Probably the right thing to do, now
that `recomputeOnce` is no longer necessary, is to:
  1. Fix `Em.computed.sort` to not use `recomputeOnce`;
  2. Fix `Em.arrayComputed` to return a proxy to the cached value;
  3. Teardown array observers when the deps count hits 0.

This will give reduce computed properties semantics much closer to regular
computed properties.  There are two important points to note about this
suggestion:

1.  The cached values would still update synchronously.  This can be improved
    with a lazy array implementation and @wycats's skiplist strategy.
2.  The semantics of an array computed would change such that existing refs
    would only be kept up to date as long as the cache had not been invalidated.
    This is probably fine: the current behaviour is not promised in the
    documentation anywhere and it would be pretty bad practice to keep
    references around rather than `Ember.get`ing them.
Remove another duplicate changelog entry.
[NICETHING beta] Expose helpful vars in {{debugger}} helper body

Folk often don't know what to do within `{{debugger}}`.
This should give them a little more to work with.
Optimizes `{{#unbound if}}` and `{{#unbound unless}}`.

They no longer create useless metamorph views and generally execute
less code. There is no public API change here, just optimization.

Props to @machty for the review and help.
Merge pull request #3881 from lukemelia/unboundif

Optimizes `{{#unbound if}}` and `{{#unbound unless}}`.
Remove long deprecated Ember.Button
[BUGFIX beta] Update to latest `ember-dev`.

* Updates to the latest `ember-dev` which ensures that the `git config
  user.name` and `git config user.email` values are present before calling
  `git cherry-pick`. This is due to a recent change in git `1.8.5` which
  requires the user information be present prior to a cherry-pick.
* Updates to bundler `1.5.0.rc1` to use the following features:
  * Automatically retry 3 times (should help with gem installation failures
    in Travis).
  * Install gems in parallel (use 4 workers by default).
* Forces each branch to be tested independently (prevents a resource
  contention issue when checking out another branch in the same test
  run).
* Travis accidentally reverted to using an older git version that does not
  support the `--points-at` flag that we are using in `ember-dev`.  This
  forces the system to upgrade to the latest version.

Travis ticket for reference:
  https://github.com/travis-ci/travis-ci/issues/1710.
[BUGFIX beta] Ensure features are tested by Travis.

 #3635 caused the Travis builds to use the `enableallfeatures` flag to
 enable features, but this was changed in `ember-dev` (and updated in
 Ember in #3633) to `enableoptionalfeatures`.
Merge pull request #3863 from rjackson/fix_publishing_of_builds

[BUGFIX beta] Update to the latest `ember-dev` and fix build publishing.
Merge pull request #3882 from selva-G/remove-emberbutton

Remove long deprecated Ember.button
Merge pull request #3854 from machty/better-debugger

[NICETHING beta] Expose helpful vars in {{debugger}} helper body
Merge pull request #3809 from ugisozols/remove_dup_changelog_entry

Remove another duplicate changelog entry.
move Ember.inspect to ember-metal
Merge pull request #3883 from stefanpenner/ember-inspect-to-metal

move Ember.inspect to ember-metal
Ember.computed.readOnly

Basically nearly every time one uses oneWay, one actually
want's to also ensure no data propagates back up. Rather then
constantly using `oneWay` and `readOnly` in conjunction, this
merely combines them.
Merge pull request #3879 from stefanpenner/computed-read-only

Ember.computed.readOnly
[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)
[BUGFIX beta] Ensure that each branch is bundled before building.

This resolves an issue that occurs when the `Gemfile.lock` differs
between branches.
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.