Add requirements for running on server

We need a gunicorn dependency to run the app on our servers, as well as
logstash_formatter so that it can output logs such that they can be
parsed by logstash.
Add PEP 8 to the build process
Add Travis to the build process

We're using Travis to tag releases, so the Travis YAML contains
an encrypted public_repo GitHub token for the @gds-pp-ci user.
Add nosetests to the build process