Re-enable beta (#612)

diff --git a/.travis.yml b/.travis.yml
index df2d331..7c17fc9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,8 +4,7 @@
 dart:
   # Keep this value aligned with min SDK value in pubspec.yaml and FROM line of Dockerfile
   - "2.10.2"
-  # Beta is trailing stable.
-  # - beta
+  - beta
 
 # Use Ubuntu 18.04 (Bionic Beaver) 
 # https://docs.travis-ci.com/user/reference/overview/#virtualization-environments