Running "bundle install"

I experienced the problem detailed on this stack overflow page:
[https://stackoverflow.com/questions/42334698/error-installing-json-1-8-3-with-ruby-2-4](https://stackoverflow.com/questions/42334698/error-installing-json-1-8-3-with-ruby-2-4)

Fixed by removing Gemfile.lock and rerunning `bundle install`.

