travis update removing 3.1

https://www.tidyverse.org/articles/2019/04/r-version-support/

Author: topepo

.travis.yml

@@ -8,7 +8,6 @@ sudo: true
 warnings_are_errors: false
 
 r:
-  - 3.1
   - 3.2
   - oldrel
   - release
@@ -17,7 +16,6 @@ r:
 
 matrix:
   allow_failures:
-    - r: 3.1
     - r: 3.2
 
 r_binary_packages: