Release r10k 3.0 and promote it into PE 2019.0

We have a 3.0 release of r10k queued up that we should ship as part of PE 2019.0, since it drops several old Ruby versions. There is currently a known issue on Ubuntu 18.04, where deploying a lot of environments all at once can result in a Ruby ASYNC bug, see RK-324. This should be documented as part of this release, and it should be noted that it also affects the most recent releases of the 2 series (and maybe even further back).