mysql2 gem error during Rails 4.2.4 upgrade

Error `rescue in spec': Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord). (Gem::LoadError) Solution Add the following version of mysql2 gem to Gemfile. gem 'mysql2', '~> 0.3.18' run bundle.