Make getting version from `git describe` work with Python 2.6 too.

Use subprocess.Popen() and its methods as check_output() helper method is not
available in Python 2.6.
Ensure that Git versions compare correctly with the released ones.

Append ".0" to account for 2 digit release versions and to ensure that the 2nd
commit after 1.2 doesn't compare greater than 1.2.1 release that will follow it.