Rails 4.2.3 + MySQL 5.6でutf8mb4を使う

database.yml

# config/database.yml
default: &default
  adapter: mysql2
  encoding: utf8mb4
  charset: utf8mb4
  collation: utf8mb4_general_ci

utf8mb4.rb

# config/initializers/utf8mb4.rb
require 'active_record/connection_adapters/abstract_mysql_adapter'

module ActiveRecord
  module ConnectionAdapters
    class AbstractMysqlAdapter
      NATIVE_DATABASE_TYPES[:string][:limit] = 191
    end
  end
end