An old mutant version is specified in Gemfile.devtools

[ch1c0t]

https://github.com/dkubb/ice_nine/blob/master/Gemfile.devtools#L47

➜  ice_nine git:(master) bundle
Updating https://github.com/rom-rb/devtools.git
Updating https://github.com/mbj/mutant.git
Fetching gem metadata from https://rubygems.org/.........
Fetching version metadata from https://rubygems.org/...
Fetching dependency metadata from https://rubygems.org/..
Could not find gem 'mutant (~> 0.6.7) ruby' in https://github.com/mbj/mutant.git (at master).
Source contains 'mutant' at: 0.8.7

[mbj]

Gemfile.devtool is obsolete. This repo can / should be updated not to use it anymore.