re-enable beta (#578)

diff --git a/.travis.yml b/.travis.yml
index d04d242..957c4c2 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
   - "2.9.0"
-  # Temporarily eliding beta, because it is currently trailing stable
-  #- beta
+  - beta
 
 # Use Ubuntu 18.04 (Bionic Beaver) 
 # https://docs.travis-ci.com/user/reference/overview/#virtualization-environments