Made version.py compatible with Python 2.6 (and maybe older)

Thanks for your package, changaco!

Recently, I integrated it into our CMS project and it seems to work fine. Our CI discovered an issue with running the tests on Python 2.6 caused by using subprocess.check_output . 

The commit will not change functionality (i hope), but just establishes support for 2.6 and maybe older versions. I can't tell for sure if there are any more issues, but our tests using several API calls run fine with this.

Maybe you have another solution, please let me know.

Thanks,