Fedora 21 support

As a note, `rosdep` requires that the OS version number be a string (similar to the way Bloom does [1]). All of Fedora 21+ will be indexed in this way, as there are no longer codenames for each release [2].

Unfortunately, Fedora 21 shipped with `rosdep` 0.10.32, and won't function correctly with anything less than 0.10.33. An update to Fedora 21 is currently pushing to stable to introduce 0.10.33 [3].

If anyone has `rosdep` resolution errors citing a missing `rosdep` key for os version "Twenty", please update to `rosdep` 0.10.33 to resolve the problem.

As with Fedora 20 support in the distribution.yaml, I will use workaround repos for those packages which have not generated a `.spec` for f21.

Thanks,

--scott

[1] https://github.com/ros/rosdistro/issues/6435#issuecomment-65871198
[2] https://github.com/ros-infrastructure/rosdep/pull/349
[3] https://admin.fedoraproject.org/updates/FEDORA-2014-16190/python-catkin_lint-1.3.8-1.fc21,python-rosdep-0.10.33-1.fc21,python-bloom-0.5.14-1.fc21,python-rosdistro-0.3.7-1.fc21
As long as @cottsay is handling the release repository forking and working around re-blooming them, I'm ok with merging this.

@tfoote @dirk-thomas are there other reasons not to allow this?