Fixed ArgumentError on redcarpet 3.1.2.

Fixed redcarpet version to 3.1.2. It seems they changed the API on a bugfix version.
This isn't quite cool in a world of semver, so just pin the current version.

The problem looked like this:
```bash
$ marktag -f - --format=2 --excmd=pattern --fields=nksSa --sort=no --append=no --language-force=markdown --markdown-kinds=h index.md 

/var/lib/gems/1.9.1/gems/marktag-0.0.3/lib/marktag.rb:46:in `header': wrong number of arguments (2 for 3) (ArgumentError)
        from /var/lib/gems/1.9.1/gems/marktag-0.0.3/lib/marktag.rb:17:in `render'
        from /var/lib/gems/1.9.1/gems/marktag-0.0.3/lib/marktag.rb:17:in `run'
        from /var/lib/gems/1.9.1/gems/marktag-0.0.3/bin/marktag:3:in `<top (required)>'
        from /usr/local/bin/marktag:23:in `load'
        from /usr/local/bin/marktag:23:in `<main>'
```