Simplify Demo Environment Deployment

This change simplifies the demo environment, currently on Heroku.

Before a feature branch had to be rebased on top of a demo branch with environment specific changes. This change creates a specific demo environment so that feature branches can just be pushed to Heroku.

`/usr/share/dict/words` is not available on Heroku so in the demo environment only we use a much simpler dictionary when checking password entropy.

I've modified `config/unicorn.rb` so that it if it can't find `/etc/govuk/unicorn.rb` it will use Heroku's [suggested unicorn config](https://devcenter.heroku.com/articles/rails-unicorn). Is there a way to test that this change won't break production deployments?

On Heroku we are excluding the test, development and extract groups when bundling, so there are no issues with trying to install sqlite.

Finally the demo environment is setup to send email via a free [Mandrill](http://mandrill.com/) service. I have no preference which service we use in demo but if someone does this can be changed to use something else.