Bump R version to 3.5

Author: jgabry

DESCRIPTION

@@ -33,7 +33,7 @@ License: GPL (>=3)
 URL: https://mc-stan.org/loo/, https://discourse.mc-stan.org
 BugReports: https://github.com/stan-dev/loo/issues
 Depends:
-    R (>= 3.1.2)
+    R (>= 3.5)
 Imports:
     checkmate,
     matrixStats (>= 0.52),