Various enhancements

[Xav83]

• Corrects the references to Python version in the example of the documentation
• Enhances the supports of some CIs job_id variables
  • First step to treat the issue circleci not supported #143

[Xav83]

Hi @eddyxu,

I hope you don't mind me already creating another Pull Request ^^

I am currently using cpp-coveralls for one of my projects, and I would like to help to make it better, for me as for others 🙂
So, if you are fine with it, I will create some others Pull Requests in the near future.

For this Pull Request, I attempted to provide a better the support of Circle CI, as there is an issue on it (#143).
I will probably try to create another Pull Request, for other CIs. To do so, I will inspire myself of node-coveralls project, and more speciffically of this file where the environment variable for each CIs are specified. 😉

[Xav83]

gentle ping @eddyxu 🙂