Update spec removing support for 1.8.7

Author: JonRowe

spec/rspec/rails/assertion_adapter_spec.rb

@@ -26,9 +26,7 @@
     expect(methods).not_to include("message")
   end
 
-  # In Ruby <= 1.8.7 Object#methods returns Strings instead of Symbols. They
-  # are all converted to Symbols to ensure we always compare them correctly.
   it 'does not leak TestUnit specific methods into the AssertionDelegator' do
-    expect(methods.map(&:to_sym)).to_not include(:build_message)
+    expect(methods).to_not include(:build_message)
   end
 end