Rails 5.0 and 5.1 already reached end-of-life, and 5.2 will reach it next week. Time to drop support for them.
Source: https://guides.rubyonrails.org/maintenance_policy.html#severe-security-issues
Additionally, removed gemfiles/sass_3_4.gemfile, which was a leftover from https://github.com/thoughtbot/administrate/pull/1197