Show warnings raised when running tests

[meshy]

This requires an update of pytest and some related libraries, and should aid the transition to Django 2.0 when the time comes.

• Rebase once #67 is merged. #68 has been rolled into this.

[meshy]

@maxpeterson any chance I can get a merge on this and #66 as a precursor to adding django 2.0 support?

[maxpeterson]

@meshy I have added you as maintainer

[meshy]

Thank you! :)