Add support to specify bower version via BOWER_ENV

Now, you can specify bower version to install.

```console
$ heroku config:set BOWER_VERSION=1.3.1
$ git push heroku master
Fetching repository, done.
Counting objects: 1, done.
Writing objects: 100% (1/1), 200 bytes | 0 bytes/s, done.
Total 1 (delta 0), reused 0 (delta 0)

-----> Deleting 3 files matching .slugignore patterns.
-----> Fetching custom git buildpack... done
-----> Ruby app detected
-----> Compiling Ruby/Rails
-----> Using Ruby version: ruby-2.1.1
-----> Installing dependencies using 1.5.2
(snip)
-----> Writing config/database.yml to read from DATABASE_URL
-----> Using Node.js version: 0.10.21
-----> Installing JavaScript dependencies using Bower 1.3.1
(snip)
```

Specifiable bower versions are listed on http://heroku-buildpack-ruby-bower.s3-website-us-east-1.amazonaws.com/ .

Close #21