Fix test failure on rubinius-3

Looks like the expected exception message on NoMethodError was too specific
and didn't match the variation that Rubinius 3 provides.

See: https://travis-ci.org/ms-ati/docile/jobs/544872700#L569

spec/docile_spec.rb

@@ -196,7 +196,7 @@ describe Docile do
 
         it "raises NoMethodError" do
           expect { subject.call(5) }.
-            to raise_error(NoMethodError, /undefined method `at' /)
+            to raise_error(NoMethodError, /method `at' /)
         end
 
         it "removes fallback instrumentation from the DSL object after block" do