build v3.1, master, and PRs only from now on

Developers should be using PRs instead of plain branches.

.travis.yml

@@ -1,6 +1,11 @@
 language: java
 sudo: false
 
+branches:
+  only:
+  - master
+  - v3.1
+
 
 before_cache:
   - rm -f $HOME/.gradle/caches/modules-2/modules-2.lock