utf8mb4で動かすために必要。create_tableメソッドで'ROW_FORMAT=DYNAMIC'がデフォルトで指定されるようにしておく。http://qiita.com/kamipo/items/101aaf8159cf1470d823

ar_innodb_row_format.rb

ActiveSupport.on_load :active_record do
  module ActiveRecord::ConnectionAdapters
    class AbstractMysqlAdapter
      def create_table_with_innodb_row_format(table_name, options = {})
        table_options = options.merge(:options => 'ENGINE=InnoDB ROW_FORMAT=DYNAMIC')
        create_table_without_innodb_row_format(table_name, table_options) do |td|
          yield td if block_given?
        end
      end
      alias_method_chain :create_table, :innodb_row_format
    end
  end
end

ActiveRecord::StatementInvalid: Mysql2::Error: Index column size too large. The maximum column size is 767 bytes.: CREATE UNIQUE INDEX unique_schema_migrations ON schema_migrations (version)

エラーへの対応