Rspec mocks stub replacement

While I was using rspec3 as my basis I was getting:

Using `stub` from rspec-mocks' old `:should` syntax without explicitly enabling the syntax is deprecated. Use the new `:expect` syntax or explicitly enable `:should` instead. Called from /root/mutant/spec/unit/mutant/matcher/methods/singleton_spec.rb:43:in `block (2 levels) in <top (required)>'.

This pull request altered the syntax and eliminated that deprecation warning. 

Not sure it is correct, so willing to adjust. 
No problem. I struggled with that myself given I introduced a longer first line of each pair, so I expected some guidance.