Add environment-specific config

We need to change application configuration based on the environment.

This change ties in with a deployment change in pp-deployment.
Include Stagecraft and Backdrop status in /_status

When requesting /_status, I'd like to know whether the dependent
apps for this app are running fine.
Merge pull request #3 from alphagov/better-status-endpoint

Add Stagecraft and Backdrop status to _status endpoint
Add requests to requirements.txt
Add Travis badge to README