Revert "Remove connection method definition, since it's called just once."

Restore the connection method which was added so it can be overridden.

This reverts commit e428c75d2b.

activerecord/lib/active_record/migration.rb

@@ -339,6 +339,10 @@ module ActiveRecord
         self.verbose = save
       end
 
+      def connection
+        ActiveRecord::Base.connection
+      end
+
       def method_missing(method, *arguments, &block)
         arg_list = arguments.map(&:inspect) * ', '
 
@@ -346,7 +350,7 @@ module ActiveRecord
           unless arguments.empty? || method == :execute
             arguments[0] = Migrator.proper_table_name(arguments.first)
           end
-          Base.connection.send(method, *arguments, &block)
+          connection.send(method, *arguments, &block)
         end
       end
     end