Avoid calling superclass.method

...in favour of using a lambda.

This addresses #2008

But really, any third party code which causes problems with the original
code is probably doing something a bit dodgy (e.g. overriding the `method` method on an
object).

Author: dgmstuart

lib/rspec/core/example_group.rb

@@ -409,7 +409,7 @@ def self.set_it_up(description, *args, &example_group_block)
 
         @metadata = Metadata::ExampleGroupHash.create(
           superclass_metadata, user_metadata,
-          superclass.method(:next_runnable_index_for),
+          lambda { |file| superclass.next_runnable_index_for(file) },
           description, *args, &example_group_block
         )
         ExampleGroups.assign_const(self)