Revert "Change to supports_atomic_switch? behavior: raise error instead ...

While it is a good idea to raise an error in cases the atomic switch bug
could happen, this forces users affected by it to port back this
change in all the migrations created using earlier versions of LHM.

This reverts commit d9893a3f3b5a7eb28d3f1aead4c75f68e9fa7b91. So users
still get a warning when trying to do it.

Conflicts:
	lib/lhm/invoker.rb
	lib/lhm/sql_helper.rb