Force dependency on Redcarpet master.

This gains readable anchor tags which look like "#cookbooks", instead
of ones which look like "#toc_37". (Redcarpet's with_toc_data
improvements.)

This points at my personal Redcarpet fork, to guard against GitHub's
security hole, where attackers can change a pull request right before
target hits accept.
Merge pull request #145 from tjg/fix-broken-links

Force dependency on Redcarpet master.