Use consistent versioning.

Update to latest `ember-dev` and utilize a consistent version number for all builds. This means that the version comment and the `Ember.VERSION` constant will be the same.

The `/VERSION` file in each branches directory will indicate the next tagged release version along with custom metadata (`+pre` for release/beta and `+canary` for master) indicating it is a prerelease version.

See https://github.com/emberjs/ember-dev/commit/4018355f5ee9c2dd0533e78c9cfa15dd72497096 for a more complete explanation.