Add Travis testing for cloud-run branch (#539)

diff --git a/.travis.yml b/.travis.yml
index 2f4aab8..c089572 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,7 +12,7 @@
 os: linux
 
 branches:
-  only: [master, dart-29]
+  only: [master, dart-29, cloud-run]
 before_script:
   - git submodule init
   - git submodule update