Skip subclass spec with SortedSet

spec/ruby/library/set/enumerable/to_set_spec.rb

@@ -7,10 +7,12 @@ describe "Enumerable#to_set" do
     {a: 1, b: 2}.to_set.should == Set[[:b, 2], [:a, 1]]
   end
 
-  it "allows passing an alternate class for Set" do
-    sorted_set = [1, 2, 3].to_set(SortedSet)
-    sorted_set.should == SortedSet[1, 2, 3]
-    sorted_set.instance_of?(SortedSet).should == true
+  ruby_version_is ''...'3.0' do
+    it "allows passing an alternate class for Set" do
+      sorted_set = [1, 2, 3].to_set(SortedSet)
+      sorted_set.should == SortedSet[1, 2, 3]
+      sorted_set.instance_of?(SortedSet).should == true
+    end
   end
 
   it "passes down passed blocks" do