chore: enable tests in node v4

Author: gajus

.travis.yml

@@ -1,10 +1,14 @@
 language: node_js
 node_js:
-    - 6
-    - 5
+  - 6
+  - 5
+  - 4
+before_install:
+  - npm config set depth 0
+  - npm install --global npm@3
 notifications:
-    email: false
+  email: false
 sudo: false
 script:
-    - npm run test
-    - npm run lint
+  - npm run test
+  - npm run lint