use dist:bionic = ubuntu 18.04

Author: erossignon

.travis.yml

@@ -43,8 +43,8 @@ matrix:
     - os: osx
 addons:
   apt:
-    sources:
-    - ubuntu-toolchain-r-test
+    # sources:
+    # - ubuntu-toolchain-r-test
     packages:
     - g++
     - libtbb2