You have already activated rack 1.6.0, but your Gemfile requires rack 1.6.4

Similar to problem with rack 1.3.2. You have already activated rack 1.3.2, but your Gemfile requires rack 1.2.3 -- I'm experiencing You have already activated rack 1.6.0, but your Gemfile requires rack 1.6.4 when attempting to run Rails (4.2) in production with Puma and Nginx.

bundle update rake nor rm Gemfile.lock && bundle install seem to help, the only solution I have so far is manually changing rack (1.6.4) to rack (1.6.0) in Gemfile.lock.

via Chebli Mohamed