Add Ruby 2.4 to README as supported version
Add Ruby 2.4 to README as supported version

benbalter/octokit.rb,LizzHale/octokit.rb,JuanitoFatas/octokit.rb,octokit/octokit.rb,LizzHale/octokit.rb,octokit/octokit.rb,JuanitoFatas/octokit.rb,pvdb/octokit.rb,pvdb/octokit.rb,benbalter/octokit.rb