Following the steps in the README, I added added the gem to my Gemfile then ran bundle install
# bundle install
Fetching gem metadata from https://rubygems.org/...........
Fetching gem metadata from https://rubygems.org/.
Could not find gem 'solidus_user_roles' in any of the gem sources listed in your Gemfile.
Following the steps in the README, I added added the gem to my Gemfile then ran bundle install