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)
Switch to MD5 from SHA1

See: https://github.com/ansible/ansible/commit/f1267c0b053e5975dc08c151530c802015902242
Merge branch 'feature/hmc/production-ami' into develop