update the hash of 3.1.1 to reflect changes to Dockerfile in caching and version pinning

library/r-base

@@ -2,4 +2,5 @@
 
 latest: git://github.com/rocker-org/rocker@105fca2b42051c5cd0894da31839c99a9b7de320 debian/r-base
 3.1.2: git://github.com/rocker-org/rocker@105fca2b42051c5cd0894da31839c99a9b7de320 debian/r-base
-3.1.1: git://github.com/rocker-org/rocker@9786da44e4451c84b70c25c71839e2270333785b debian/r-base
+3.1.1: git://github.com/rocker-org/rocker@2d09bbed2a9d180ecb96282fc8458c747039be3 debian/r-base
+