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
Merge pull request #2953 from prathamesh-sonpatki/remove-deprecatio-warning

Remove deprecation warning caused by use of failure_message_for_should
use the newest ruby 2.1 to test with
rubygems 2.2.2 is out now
clone rubygems via https
Use git sha as part of extension directory for git referenced gems.

Fixes #2947