Workaround for broken nose_xunitmp

When nose 1.3.1 was released a few weeks ago, it removed a _forceUnicode() method that the nose_xunitmp plugin depended on. This causes our reporting to fail at the end of a series of tests. I forked the package and made a minor change to fix this missing method, so this PR is just to point to the new plugin location in requirements.txt.