Add Melodic to list

[allenh1]

There's just a small issue after this one, and I think we'll do a patch release.

[allenh1]

$ superflore-check-ebuilds --log-file ~/melodic-ros.txt --ros-distro melodic --pkgs ros
>>>> testing gentoo package integrity
>>>> Running container with command string 'bash -c 'emaint sync -r ros-overlay &>> /home/allenh1/melodic-ros.txt && emerge ros-melodic/ros &>> /home/allenh1/melodic-ros.txt''...
>>>> Docker container exited.
>>>> Log file: '/home/allenh1/melodic-ros.txt'
>>>>   'ros-melodic/ros': building