chore: enable tests for node v4

Author: gajus

.travis.yml

@@ -2,6 +2,10 @@ language: node_js
 node_js:
   - 6
   - 5
+  - 4
+before_install:
+  - npm config set depth 0
+  - npm install --global npm@3
 notifications:
   email: false
 sudo: false