remove hard dependency on `forem-redcarpet`

The `redcarpet` gem does not build on JRuby, and this dependency effectively prevents JRuby support for Forem. There are other formatter alternatives, and I have just released [forem-kramdown](https://github.com/phlipper/forem-kramdown) as well. 

I would add a note in the README, but there is already a link to the [Formatters wiki page](https://github.com/radar/forem/wiki/Formatters), where I have added a link to `forem-kramdown`. I would be happy to add an additional note if necessary.