追記: Rails4.2.5以降では以下の問題は解消しています Error log: but the gem is not loaded??? な、なぜ... /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/connection_specification.rb:177:in `rescue in spec': Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to yo ur Gemfile (and ensure its version is at the minimum requi