Heptapod CI: job to produce and push heptapod-rails tarballs
Closes: #339 (closed)
We could tighten things a bit more (host checking etc), but we need to publish this almost immediately and then test it with a tag.
The produced tarballs will be available as heptapod-rails-[VERSION_OR_HG_SHA].tar.gz
on https://download.heptapod.net/src