Filter "outputs has no custom" warnings for tests

If you run the tests using nosetests, you see this being printed out:

    ...
    /home/travis/build/spotify/luigi/luigi/task.py:471: UserWarning: Task MyDynamicTask(x=44) without outputs has no custom complete() method
    warnings.warn("Task %r without outputs has no custom complete() method" % self)
    /home/travis/build/spotify/luigi/luigi/task.py:471: UserWarning: Task MyDynamicTask(x=45) without outputs has no custom complete() method
    warnings.warn("Task %r without outputs has no custom complete() method" % self)
    /home/travis/build/spotify/luigi/luigi/task.py:471: UserWarning: Task MyDynamicTask(x=46) without outputs has no custom complete() method
    warnings.warn("Task %r without outputs has no custom complete() method" % self)
    ...

This patch silences those warnings.