r build-pack: when using ppa install r-recommended

Attempt at fixing https://github.com/jupyterhub/mybinder.org-deploy/issues/1308

repo2docker/buildpacks/r.py

@@ -250,6 +250,7 @@ class RBuildPack(PythonBuildPack):
                     apt-get update && \
                     apt-get install --yes r-base={R_version} \
                          r-base-dev={R_version} \
+                         r-recommended={R_version} \
                          libclang-dev && \
                     apt-get -qq purge && \
                     apt-get -qq clean && \