Docker build broken

[adammcmaster]

Installing listen 3.1.1

Gem::InstallError: listen requires Ruby version ~> 2.2.
Installing rspec 3.4.0
Installing virtus 1.0.5
Installing http-cookie 1.0.2
An error occurred while installing listen (3.1.1), and Bundler cannot continue.
Make sure that `gem install listen -v '3.1.1'` succeeds before bundling.
Removing intermediate container e18cace009cb
The command '/bin/sh -c bundle install' returned a non-zero code: 5

[parrish]

That's coming from guard which is specified as a development dependency, so I'm not sure why this is an issue.

Development dependencies aren't installed by default and aren't activated when a gem is required.

Failing that, we can just bump to 2.2.