chore(travis): whitelist greenkeeper branches

greenkeeper[bot] · greenkeeper[bot] · commit 19d1400382dd · 2017-04-15T13:49:38.000Z
https://greenkeeper.io/
diff --git a/.travis.yml b/.travis.yml
@@ -6,6 +6,7 @@ node_js:
 branches:
   only:
     - master
+    - /^greenkeeper/.*$/
 
 env:
   - CXX=g++-4.8
