Improved build matrix

I added Python 3.3 and PyPy to the build matrix. I understand why one would not want to support Python 3.2 or 2.6 but there's no reason not to support PyPy and Python 3.3 (and PyPy 3 once it will implement 3.3 or above)
I used travis_retry in order to avoid build failures due to network issues.
The build will now install the latest patch version of each Django minor version.
The matrix will now fast finish since it's large.