make an expectation _about_ 'matcher == VALUE' for linter

my prior '.==(VALUE)' solution passed linting on modern rubies, but
apparently on older rubies it still caused a message like

    possibly useless use of == in void context

This added expectation doesn't _entirely_ belong, but it's not
completely out of place, and it convinces the linter/parser not to
worry.

spec/rspec/matchers/built_in/operators_spec.rb

@@ -245,7 +245,7 @@ def method_missing(name, *args, &block)
 
     it "is described correctly" do
       matcher = 7.should
-      matcher.==(7)
+      expect(matcher == 7).to be_truthy
       expect(matcher.description).to eq("== 7")
     end
   end
@@ -268,7 +268,7 @@ def method_missing(name, *args, &block)
 
     it "is described correctly" do
       matcher = 7.should_not
-      matcher.==(8)
+      expect(matcher == 8).to be_falsey
       expect(matcher.description).to eq("== 8")
     end
   end