Fix misleading package description.

Testing Done:
$ PANTS_DEV=1 ./pants setup_py src/python/pants:pants-packaged
*** Running pants in dev mode from /home/jsirois/dev/3rdparty/jsirois-pants/src/python/pants/bin/pants_exe.py ***
WARNING!  /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants.FT6ETM/src/pants/docs/conf.py is source but does not belong to a package!
WARNING!  /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants.FT6ETM/src/pants/docs/pagerank.py is source but does not belong to a package!
Running packager against /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants-0.0.19
Writing /home/jsirois/dev/3rdparty/jsirois-pants/dist/pantsbuild.pants-0.0.19.tar.gz
$ cd dist/
$ tar -xzf pantsbuild.pants-0.0.19.tar.gz
$ cd pantsbuild.pants-0.0.19/
$ python setup.py --description
A bottom-up build tool.

Bugs closed: 381

Reviewed at https://rbcommons.com/s/twitter/r/725/