Update release-notes.md
Update release-notes.md

Changed the year to 2019 for 3.1.2, 3.0.8, and 2.2.15. Will set the actual date when release happens.
thaJeztah/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io