Update URL to a deterministic one.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

This guarantees for the package feeds that
the mk files will always be available for all packages.

Will need to see about external-feed Python packages
a bit later.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

Flask is a microframework for Python based on Werkzeug, Jinja 2 and
good intentions. And before you ask: It.s BSD licensed!

Signed-off-by: Daniel Golle <daniel@makrotopia.org>