Add .travis.yml to automatically run Travis-CI on each commit / pull
request

To avoid accidentally breaking the testsuite in the future. For this to
work, you need to setup the "Travis" service hook under
https://github.com/gabrielhurley/django_openstack_auth/settings/hooks.
For that you need to register at https://travis-ci.org and sync github
repos / enable travis for django_openstack_auth.