Today when updating my ruby to 2.2.5 I faced some troubles with updating my bundle. The error was

  # sudo gem install json 

  Building native extensions.  This could take a while...
  ERROR:  Error installing json:
    ERROR: Failed to build gem native extension.

After some investigation I found a way to install json and finally got SimpleCov and Mutant running.

  brew install coreutils
  gem install json


have fun and happy hacking