apt-pinning for R install

Uses apt-pinning to install the latest version of R from debian:unstable.  (The previous strategy of installing the latest version of R from CRAN mirror could create dependency problems resulting in a failed build).

library/r-base

@@ -1,6 +1,6 @@
 # maintainer: "Carl Boettiger" <[email protected]> (@cboettig)
 # maintainer: "Dirk Eddelbuettel" <[email protected]> (@eddelbuettel)
 
-latest: git://github.com/rocker-org/rocker@105fca2b42051c5cd0894da31839c99a9b7de320 debian/r-base
-3.1.2: git://github.com/rocker-org/rocker@105fca2b42051c5cd0894da31839c99a9b7de320 debian/r-base
+latest: git://github.com/rocker-org/rocker@56cb59cc262e9d2c02f9e11cfacab65f0b659c5f r-base
+3.1.2: git://github.com/rocker-org/rocker@56cb59cc262e9d2c02f9e11cfacab65f0b659c5f r-base