Add r-base, an official container for R

The r-base image is maintained by the [rocker-org](https://github.com/rocker-org/rocker) project. This would add support for an official container for R, as suggested in https://github.com/rocker-org/rocker/issues/16
list both maintainers
fix git url (use `://` not `@`)
adds 3.1.1 tag, builds from debian:jessie, removes apt-get upgrade
update hash to point to the updated Dockerfile
adds tag for R 3.1.2 as discussed in PR #310
update the hash of 3.1.1 to reflect changes to Dockerfile in caching and version pinning
List maintainers on separate lines and drop 3.1.1
Merge pull request #310 from rocker-org/master

Add an official image for r-base (R language)