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,
You have already activated rack 1.6.0,
you need to uninstall one version of rack which is not required.
Do this please
gem uninstall rack -v 1.6.0
Reference: How to force rack to work around the usual "You have already activated rack..." bug?