Change mock to double to make tests pass

Mock have been deprecated in new version of rspec
See compare here
https://github.com/rspec/rspec-mocks/compare/v3.0.0.beta1...v3.0.0.beta2
Commit ef4aa3b
Add possibility to change stepping_thread_group name

Kept default name in case someone is using it without name
Merge pull request #34 from Typeform/master

Change mock to double to make tests pass