Ruby on Rails

I have 5 Rails 5 "canery apps' that in the past run bundle cleanly, but yesterday things changed.

I have two repos (rt_demo_app5, rt_first_app5) that have run bundle cleanly (after adding rack

master to Gemfile) and 3 that do not (hello_app5, toy_app5, sample_app_5_0_rt) even after

adding the rack master to Gemfile.

For the ones that do not run bundle cleanly, the error message is (before adding rack master to Gemfile):

  Bundler could not find compatible versions for gem "rack":       In Gemfile:         rails (>= 0) ruby depends on           actionpack (= 5.0.0.alpha) ruby depends on             rack (~> 2.x) ruby       Could not find gem 'rack (~> 2.x) ruby', which is required by       gem 'actionpack (= 5.0.0.alpha) ruby', in any of the sources.
 

Example Public repo: https://bitbucket.org/jasnow/sample_app_5_0_rt

I checked rubygems.org and latest rack is at 1.6.4 so unclear how to get 2.x.

So how do I change my Gemfile to find "rack 2.x"?

Thanks,

Al