Update .gitignore

When using buildout with mr. developer set to checkout the source from github, there are errors caused by the created sockjs_tornado.egg-info when attempting to rebuild the buildout. The repo has uncommitted changes.

You can see the fix here:
http://buildoutcoredev.readthedocs.org/en/latest/issues.html#dirty-packages

This change will allow buildout to work as expected.