Upgrade Travis environment from xenial to bionic

With the switch to bionic, Travis disabled a lot of things by default
that might improve test image startup times.
diff --git a/.travis.yml b/.travis.yml
index 7abf8d7..4996e78 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,4 @@
+dist: bionic
 language: node_js
 node_js:
   - "10"