Make texlive deps specific to Fedora 18

Those texlive packages don't exist in Fedora < 18, so we should not attempt to install them. A missing key error is more appropriate than a package not found error in this case. Fedora 17 actually doesn't HAVE some of the necessary fonts available, even under different package names.
Merge pull request #422 from smd-ros-devel/master

Make texlive deps specific to Fedora 18