travis: Use bionic build environment

https: //docs.travis-ci.com/user/reference/bionic/

Change-Id: I8f622506329ee97ff3555c2a43a2165303d9ae82
diff --git a/.travis.yml b/.travis.yml
index 5230b46..182e006 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,5 @@
 sudo: required
+dist: bionic
 
 language: ruby