README.rst: Fix title underline too short
README.rst: Fix title underline too short

This prevents the description from rendering nicely on PyPI.

Merge this and then do `python setup.py register` to update the
description.

See
https://bitbucket.org/pypa/pypi/pull-request/60/fix-bb-214-make-rst-rendering-not-fail-for/diff

jcfr/girder,opadron/girder,essamjoubori/girder,adsorensen/girder,girder/girder,salamb/girder,jbeezley/girder,chrismattmann/girder,salamb/girder,Xarthisius/girder,Xarthisius/girder,opadron/girder,essamjoubori/girder,sutartmelson/girder,girder/girder,kotfic/girder,kotfic/girder,essamjoubori/girder,jbeezley/girder,opadron/girder,RafaelPalomar/girder,Xarthisius/girder,Kitware/girder,adsorensen/girder,opadron/girder,RafaelPalomar/girder,Kitware/girder,adsorensen/girder,kotfic/girder,adsorensen/girder,Kitware/girder,essamjoubori/girder,sutartmelson/girder,Xarthisius/girder,chrismattmann/girder,Xarthisius/girder,RafaelPalomar/girder,msmolens/girder,data-exp-lab/girder,jbeezley/girder,data-exp-lab/girder,salamb/girder,data-exp-lab/girder,chrismattmann/girder,kotfic/girder,msmolens/girder,sutartmelson/girder,msmolens/girder,kotfic/girder,sutartmelson/girder,manthey/girder,RafaelPalomar/girder,msmolens/girder,manthey/girder,data-exp-lab/girder,chrismattmann/girder,chrismattmann/girder,girder/girder,Kitware/girder,sutartmelson/girder,girder/girder,data-exp-lab/girder,opadron/girder,salamb/girder,adsorensen/girder,manthey/girder,jbeezley/girder,jcfr/girder,RafaelPalomar/girder,manthey/girder,salamb/girder,msmolens/girder,jcfr/girder,jcfr/girder,jcfr/girder,essamjoubori/girder