text: Title: Test factory_boy against nightly Python builds
  Body: #### The problem
  ``factory_boy`` should be tested against the latest Python development.
  
  #### Extra notes
  There might be some issues with configuring Travis-CI with python nightly.
  
  #### Checklist:
  
  - [ ] Add the target to ``tox.ini``
  - [x] Add that target to ``.travis.yml``
  - [ ] Maybe think of a way to not mark a PR as failing if it is only failing against nightly?