Apply change made to version-at-commit.sh to arvados_version.py

Apply change made to version-at-commit.sh to arvados_version.py

Brings the Python code in line with 3a122933b4

This strips leading "development-" from the generated version string.
Version strings are derived from tags, which are usually just the bare
version number.  However, in order to get development versions to
interact properly with release branches, we sometimes need to apply
version tags with a "development-" prefix to the main branch.

no issue #

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>