Remove deprecation warning caused by use of failure_message_for_should

- RSpec has replaced failure_message_for_should with failure_message in
  https://github.com/rspec/rspec-expectations/commit/bfe92020d5569350c1382235feb9b113907fa2db
- This change removes deprecation warning caused by https://github.com/rspec/rspec-expectations/commit/cd0d347d3115f8aad801e6dc8fe2a311d75626bd#diff-a51020971ade2c87f1d5b93f20d711c7R207