Force non-ANSICON environment.

On appveyor we install ANSICON.

Author: myronmarston

spec/rspec/core/configuration_spec.rb

@@ -1036,6 +1036,8 @@ def metadata_hash(*args)
           end
 
           context "with ANSICON NOT available" do
+            around { |e| with_env_vars('ANSICON' => nil, &e) }
+
             before do
               allow_warning
             end