Heroku deploy step added to wercker

#### What's this PR do?
This provides a staging server for this repo. I added a Heroku deploy step to our wercker setup that will auto-deploy any commits to the `dev` branch.

`wercker.yml`
This adds the SSH key needed for the Heroku deploy step. (see http://blog.wercker.com/2014/01/15/best-practices-heroku-deploy-step.html)

`Procfile`
Heroku will use this to start the app.

#### What are the relevant tickets?
SMS-65