Add Gemfile mutant 0.11.22

[dkubb]

Summary

This branch adds mutant mutant 0.11.22 to the Gemfile. It also adds configuration and settings to ensure the code is always tested with mutant.

TODO

• [x] Extract mutant addition to separate branch
  • [x] Fix alive mutations, until full mutation coverage
  • [x] Change mutant to use `--since HEAD~1' for faster feedback

Dependencies

• [x] #27