update Travis CI to use bionic

Author: eddelbuettel

.travis.yml

@@ -2,7 +2,7 @@
 
 language: c
 sudo: required
-group: edge
+dist: bionic
 
 before_install:
   - curl -OLs https://eddelbuettel.github.io/r-travis/run.sh && chmod 0755 run.sh
@@ -20,4 +20,4 @@ after_failure:
 notifications:
   email:
     on_success: change
-    on_failure: change
+    on_failure: change