Modules should not have subclasses.

	When refining a module, the module was set to the superclass of its refinement,
	and a segmentation fault occurred.
	The superclass of the refinement should be an iclass of the module.
	[ruby-core:83617] [Bug #14070]

	The superclass of a refinement should have BasicObject as its ancestor.

	Otherwise, VM_ASSERT(callable_method_entry_p(cme)) in
	prepare_callable_method_entry() fails if VM_CHECK_MODE is 2.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@62233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e