cron.runPropellor now runs propellor, rather than using its Makefile. This is more robust.

Particularly, the Makefile would try to build propellor if there were
updates, but if this build failed because a bad commit was pushed, it would
abort w/o pulling any new fixed commits.

Anyway, it's better to move away from using the makefile when possible as
it's just for bootstrapping, and also I would like to make "make" not
default to running propellor as that can be confusing behavior.

(I can't change that yet; needed for upgrades. Should be fine to change
after several more releases, or maybe a year..)