Allow looser version of nose

TravisCI provides `nose` already installed:

- http://docs.travis-ci.com/user/languages/python/#Pre-installed-packages

However it's now at a later version and causes our tests to fail:

    pkg_resources.VersionConflict: (nose 1.3.4 (/home/travis/virtualenv/python2.7.8/lib/python2.7/site-packages), Requirement.parse('nose==1.3.1'))