Update entries for sdformat and gazebo

The update for gazebo and sdformat packages. Some details:

 * I did not see any entry using Trusty or Saucy. Should I leave them out?
 * I included the sdformat package in debian SID. It is in [debian NEW queue right now](https://ftp-master.debian.org/new.html) and I hope to get it in before the end of sync in Trusty.
 * In sdformat for fedora, I did not see entries for 19 or 20. 19 is specially fun since it is called "Schrödinger's Cat", I don't know if the funky chars or spaces will work fine for rosdep.
 * I removed gentoo for gazebo. Too out of date.

/cc @DLu. @nkoenig, @scpeters