use updated location for rstudio docker image

We've moved the R Docker images to a Docker organization ([rocker](https://registry.hub.docker.com/repos/rocker/), corresponding github repo now it's own github org as well: [rocker-org](https://github.com/rocker-org/rocker)) and shortened the image names. Though the original images won't disappear from the Hub immediately, figured I might as well update the paths.