Fix tests that vary urls.py

Create a mixin class that can be used for tests that customize urls.py
to force django to reload it, so that they don't break other tests.
Do a bundle install before trying to use rake, so that we have all the prerequisites when running tests
Merge pull request #92 from edx/fix/cale/url-tests

Fix tests that vary urls.py
Merge pull request #124 from edx/fix/cale/test-ruby-install

Do a bundle install before trying to use rake, so that we have all the p...