fix(travis): optimize config for greenkeeper

Author: csvenke

.travis.yml

@@ -1,8 +1,18 @@
 
 language: node_js
+
 node_js:
   - "node"
   - "8"
+
+notifications:
+  email: false
+
 script:
   - npm run lint
-  - npm run test
+  - npm run test
+
+branches:
+  only:
+    - master
+    - /^greenkeeper.*$/