text: Title: Check support for Python 3.6
  Body: With Python 3.6 out, we should make sure that it is properly supported by factory_boy.
  
  This should work out of the box; but some steps are required to check it:
  1. Add Python 3.6 to https://github.com/FactoryBoy/factory_boy/blob/master/tox.ini
  2. Add that build in https://github.com/FactoryBoy/factory_boy/blob/master/.travis.yml
  3. Make a pull request, wait for the travis build, and check that it's still working.
  4. Update ``README.rst`` and classifiers at https://github.com/FactoryBoy/factory_boy/blob/master/setup.py#L57 to state that Python 3.6 is supported.
  