Don't install the KnownFailure nosetest plugin as a pkg_resources entry_point, as it can, under some circumstances, conflict with the one in IPython.  This means that the supported method to run the tests is from the tests.py script or `import matplotlib; matplotlib.test()`.
Make the KnownFailure plugin work when the tests are run in parallel
Merge pull request #2139 from mdboom/dont-install-nose-plugin

Don't install the nose KnownFailure plugin as a pkg_resources entry_point