Using Node v4 in travis CI

The test suit doesn't run in Node v5, but I'm not sure whether this is a
problem with jest or react-docgen (or one of its dependencies). Will
have to investigate more...

Author: fkling

.travis.yml

@@ -1,4 +1,4 @@
 sudo: false
 language: node_js
 node_js:
-  - "stable"
+  - "4"