Require redcarpet >= 3.1.1 for yard docs.
Require redcarpet >= 3.1.1 for yard docs.

This is needed because version 3.0.0 throws an error when trying to
generate docs.

Change-Id: I27af3c466087ea8f2e28be3bc86b411e2bf62248

attune-api/attune-ruby