Make application run in production mode

This changeset makes changes to the Django application, and application specific provisioning tasks in order to produce an Amazon Machine Image (AMI).

Summary of key changes:

- Update Ansible to 1.8.2
- Add Git as a dependency across the board
- Add `django-statsd-mozilla` to base requirements file
- Add a default cache to the production settings
- Add `POSTGIS_VERSION` setting to avoid talking to DB for collectstatic
- Set an application `SECRET_KEY`
- Add `.amazonaws.com` & `treescount.nycgovparks.org` to `ALLOWED_HOSTS`
- Make deployment branch a variable (`app_deploy_branch`)
- Set this branch as the deploy branch (for easier testing)

This changeset makes changes to the Django application, and application specific provisioning tasks in order to produce an Amazon Machine Image (AMI).