update R 3.5.0 image using post-transition packages (#4473)

* update R 3.5.0 image using post-transition packages

* updated R 3.5.0 container with testing (cf #4473)

library/r-base

@@ -1,5 +1,5 @@
 # maintainer: "Carl Boettiger" <[email protected]> (@cboettig)
 # maintainer: "Dirk Eddelbuettel" <[email protected]> (@eddelbuettel)
 
-3.5.0: git://github.com/rocker-org/rocker@629c6f89d81a336a7fa6a8a18f8365f1fd2878be r-base
-latest: git://github.com/rocker-org/rocker@629c6f89d81a336a7fa6a8a18f8365f1fd2878be r-base
+3.5.0: git://github.com/rocker-org/rocker@960999dd029454202f59cf4beeaedaceecd27ec6 r-base
+latest: git://github.com/rocker-org/rocker@960999dd029454202f59cf4beeaedaceecd27ec6 r-base