no need to continue to point to 3.1.2 image, handled automatically

(as @yosifkit points out to us)

library/r-base

@@ -3,5 +3,4 @@
 
 latest: git://github.com/rocker-org/rocker@90f657c14b1ce53a523343e7792d1801d536d320 r-base
 3.1.3: git://github.com/rocker-org/rocker@90f657c14b1ce53a523343e7792d1801d536d320 r-base
-3.1.2: git://github.com/rocker-org/rocker@fa244c6a6010d41fca568d2469be681959a1f3c4 r-base