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.

Additionally, removed gemfiles/sass_3_4.gemfile, which was a
leftover from #1197

https://guides.rubyonrails.org/maintenance_policy.html#severe-security-issues
https://github.com/thoughtbot/administrate/pull/1197