Setup bower canary/beta auto release

This PR sets up a script that will run on successful build that will push to [components/ember](https://github.com/components/ember) builds for 

(branch / channel)
- master => canary
- beta => beta
- stable or release => release

This also moves the versioning responsibility into ember's build step.  This is being done manually now (as far as I am aware) 

# Pre-requisites 
It requires the following travis setup:

(from here on I'll assume user means a user with authorization to components/ember)

User generates github token by following the [guide](https://help.github.com/articles/creating-an-access-token-for-command-line-use) with the 'public_repo' OAuth scope.

User installs travis gem and runs

```sh
travis encrypt GH_TOKEN=<YOUR TOKEN> --add env.global
```

The above will modified your .travis.yml file adding a GH_TOKEN secret key that will be made available to the travis CI build (and `bin/bower_ember_build` script).  When .travis.yml is update user should add .travis.yaml to this PR.

# update bin/bower_ember_build

The user needs to set:

- Global git user email - this will affect the git graph for whichever user
- Global git user name
- `COMPONENTS_EMBER_REPO_SLUG` should be set to `components/ember` (or the like)
- `USER` should be user

These vars ensure that the repo can be successfully pushed to components/ember.
Can you tweak this to use the VERSION_STRING_PLACEHOLDER?