The std-msgs recipe is updated to 0.5.8 to be in line with
https://github.com/ros/rosdistro/blob/4551e7c32d1b34984410b79bc0ec36afe3d3d245/hydro/release.yaml.
The license string in the package.xml was tuned in 0.5.8 and the
hash of the license string is now the common one.