This changes the build slightly. If snapcraft is triggered when there is a new tagged release in the project github release page, and it's newer than the version in the Snap Store beta channel, we build that stable release. If however, the latest stable release in github releases is already the same as the Snap Store beta channel, then we build the tip of master.
This gives a couple of advantages.
* One yaml can be used to build tip-of-git snaps, and stable releases alike
* Closing the beta channel in the Snap Store will mean the next triggered build will re-build whatever the last stable release is. This is useful to force a rebuild of the stable version in case a dependency (not that there are many) has a security issue.
We also now set the version dynamically based on the git tags.
