Add workaround comment.

So until further notice Travis uses 12.04, which complicates validating the indigo branch. In the meantime, we can try to build `indigo-devel` on top of hydro+12.04. It's possible that rosdeps will not be successfully resolved for all repos (at least ros_control should fail). If this attempt fails, I'll rollback changes to test hydro, and look for more definitive solutions to using Travis with 14.04.