Add redcarpet gem to Gemfile

When running the tests:

> cannot load such file -- redcarpet: skipping markdown tests with Tilt::RedcarpetTemplate
uninitialized constant Redcarpet: skipping markdown tests with Tilt::RedcarpetTemplate::Redcarpet2
uninitialized constant Tilt::RedcarpetTemplate::Redcarpet1::RedcarpetCompat: skipping markdown tests with Tilt::RedcarpetTemplate::Redcarpet1

This PR adds `redcarpet` to `Gemfile`, except when running on JRuby.