Make debug messages more informative

https://github.com/fs/codeclimate_ci/issues/18
Cache on this level was implemented because we call `diff` method two times and we need to wait two times longer.

What do you think about 
```ruby
    def master_gpa
      @master_gpa ||= gpa('master').gpa
    end

    def branch_gpa(branch_name)
      @branch_gpa ||= gpa(branch_name).gpa
    end