Remove instructions for yum.puppetlabs.com on Fedora 17

Not only is Fedora 17 past EOL, that repo isn’t even there anymore.
DOCUMENT-19
Added yum repos for Fedora 20
Removed Fedora 17 from supported list, added Fedora 20

I think we should hold off on purging Fedora 18 from the docs site
until we get rid of the repo for it.
Merge pull request #274 from puppetlabs/rm-old-fedora

Document 19 - Remove mentions of EOL Fedora versions