Fix gratipay.version when Git is not installed

Python raises OSError in this case. The fact is missing from check_output docs https://docs.python.org/2/library/subprocess.html#subprocess.check_output
version: Do not fail on git version check
Simplify version handling

See https://github.com/gratipay/gratipay.com/pull/2905 for discussion
Merge pull request #2905 from gratipay/gitversionfix

Fix gratipay.version when Git is not installed