(maint) Add dependency information for yard and redcarpet

Without this patch bundle exec yard server --reload doesn't work.
This is a problem because people who are developing Puppet need to be
able to access the API documentation easily and the yard server is one
such way. This patch addresses the problem by adding a dependency on
yard and redcarpet for the development group only. This will prevent
the yard and redcarpet gem from sucking in a large number of
dependencies when running in a CI environment because these environments
are expected to omit development only dependencies when bootstrapping
using bundle install --path vendor --without development