Allow Heroku to access ENV variables during asset compilation

This is required for the Devise 3.1.0 configuration introduced in this commit: https://github.com/errbit/errbit/commit/d55891c9ba2c27b40f5ea889fd8b0a92e9d9caec

If this labs addon is not added to Heroku, asset precompilation will fail. Previously, this continued to work because Heroku would compile assets at runtime, but it has been removed. See the following message:

```
       Precompiling assets failed, enabling runtime asset compilation
       Please see this article for troubleshooting help:
       http://devcenter.heroku.com/articles/rails31_heroku_cedar#troubleshooting
-----> DEPRECATIONS:
       Runtime asset compilation is being removed on Sep. 18, 2013.
       Builds will soon fail if assets fail to compile.
```

Adding a Heroku addon to solve an issue is not necessarily the best way of fixing this, but it is a way.

Thoughts?