Improved build matrix.

Added PyPy and PyPy 3.
Allowed failures for the latest django develop version.
Loosened the django version to use the latest minor version.
The matrix should fast finish since it's large.
Used travis_retry in order to avoid build failures due to networking issues.
Fixed the allow failiures section of the build.
Fixed escaping issues and exclusion issues.
Increased test verbosity.
Excluded PyPy from the build matrix for now.
Merge pull request #164 from thedrow/patch-1

Improved build matrix