Fix gratipay.version when Git is not installed
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
gratipay/gratipay.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,studio666/gratipay.com,mccolgst/www.gittip.com,gratipay/gratipay.com,mccolgst/www.gittip.com,gratipay/gratipay.com,studio666/gratipay.com,studio666/gratipay.com,gratipay/gratipay.com,studio666/gratipay.com,eXcomm/gratipay.com,eXcomm/gratipay.com