Drop support for R 3.1 (#3309)

Author: lionel-

.travis.yml

@@ -18,11 +18,11 @@ matrix:
       provider: script
       script: Rscript -e 'pkgdown::deploy_site_github(verbose = TRUE)'
       skip_cleanup: true
-  - r: oldrel
+  - r: 3.5
+  - r: 3.4
+  - r: 3.3
   - r: 3.2
     env: R_REMOTES_NO_ERRORS_FROM_WARNINGS=true
-  - r: 3.1
-    env: R_REMOTES_NO_ERRORS_FROM_WARNINGS=true
 
 # environment variables set for all builds
 env:

DESCRIPTION

@@ -17,7 +17,7 @@ Authors@R: c(
     person("RStudio", role = c("cph"))
     )
 Depends:
-    R (>= 3.1)
+    R (>= 3.2)
 Imports: 
     digest,
     grDevices,

NEWS.md

@@ -7,6 +7,9 @@ extension developers if they have relied on internals that have been changed.
 This release also sees the addition of Hiroaki Yutani (@yutannihilation) to the 
 core developer team.
 
+With the release of R 3.6, ggplot2 now requires the R version to be at least 3.2,
+as the tidyverse is committed to support 5 major versions of R.
+
 ## Breaking changes
 
 * Two patches (#2996 and #3050) fixed minor rendering problems. In most cases,