It looks like Rails 5.2.0 isn't happy with mysql2, ~> 0.3.18.

Gem::LoadError: Error loading the 'mysql2' Active Record adapter. Missing a gem it depends on? can't activate mysql2 (< 0.6.0, >= 0.4.4), already activated mysql2-0.3.21. Make sure all dependencies are added to Gemfile.

This does not appear to be relevant because 'mysql2, ~> 0.3.18' is hard set.