Enable travis build for maintable-beta (#3804)

I think it would be nice to have travis build for maintable-beta. 
That would show us some potential errornous tests or

Author: Siedlerchr

.travis.yml

@@ -60,6 +60,7 @@ after_failure:
 branches:
   only:
     - master
+    - maintable-beta
 
 # cache gradle dependencies
 # https://docs.travis-ci.com/user/languages/java#Caching