Pass $group parameter to Migration base class constructor

This patch is no longer applicable as the API has different ideas about the arguments that go into the construct... and fortunately group is now one of the parameters that is possible to put into the array of arguments variable used in the constructs of latest versions of migrate. It is therefore now built-in.

Since migrate 2.5 this hasn't been necessary, and in migrate 2.6 it will cause things to fail.

The new 2.x branch of this module creates a group for itself. And like any migrate 2.6 migration, you can use the migrate_api_alter hook to change the group if you need. See #1832736: Class registration for Migrate 2.5 or later for more details on the new branch.