Force dependency on Redcarpet master.

This fixes broken links, solving https://github.com/clojuredocs/guides/issues/143.

(It forces dependency on Redcarpet's master branch, to gain `with_toc_data` improvements.)

**Security note:** This points at my personal Redcarpet fork, to guard against GitHub's security hole where attackers can change a pull request right before target accepts it. However, *I may be an attacker*. So clojuredocs should probably reject this pull request, create its own Redcarpet fork, and point the gemfiles to it.

Verification:
- [x] `diff -r _site _site.old > diff.txt`
- [x] Verify diff with https://gist.github.com/tjg/b7c26738d279c2078604