Automate Heptapod Rails revision for requirements file
We currently retrieve the Python requirements for this image from the Heptapod Rails repo with a hardcoded tag name that has to be changed each time there's a bump of the stable branches.
This could probably become automatic, something like checking HEPTAPOD_VERSION in the heptapod-stable branch, then looking at the corresponding release/support branch (e.g., heptapod-0-19).