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
I understand and have no expectations of the Indigo package maintainers. I'll continue to manage workaround repos for Indigo, and we'll get the Fedora story right from the start in Jade so that this isn't necessary.

In theory, if I fork the release repos once and do generation for f21, a future release by that package owner would automatically generate for f21 (given that `rosrpm` generation is enabled to begin with). This means I shouldn't need to do the fork-and-generate more than once.

In any case, I have a script to do this for me :)