Automatically release patch and/or minor buildpacks

[robdimsdale]

#679 added automation to file issues to remind maintainers to release buildpacks.

This issue proposes that we go one step further and automatically release buildpacks on a schedule. We would only want to do this if the release would be a patch or minor release - we would not want to automatically ship major releases.

#649 is somewhat orthogonal - it would likely result in fewer releases whether they are released manually or automatically.