How to force rack to work around the usual “You have already activated rack…” bug?

Question

This is a common question, but none of the answers seem to solve the issue. I get the usual: You have already activated rack 1.4.1, but your Gemfile requires rack 1.3.

Answer 1

If the same error message concerning Spring brought you here, i.e. you're getting a message similar to this one:

You have already activated spring 1.4.0, but your Gemfile requires spring 1.3.6.

The solution is the same as the accepted answer:

gem uninstall spring -v 1.4.0