Make sure travis uses latest Chrome

PiperOrigin-RevId: 338139706
diff --git a/.travis.yml b/.travis.yml
index e6a4be6..73b0d68 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,8 @@
 language: dart
 
+addons:
+  chrome: stable
+
 dart:
   - dev