diff --git a/.travis.yml b/.travis.yml
index a6402cd..0954e04 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,8 @@
 language: node_js
 node_js:
+    - "4"
+    - "6"
+    - "8"
     - "9"
 install:
     - yarn
diff --git a/index.js b/index.js
index b2d2ba7..9ec8b01 100644
--- a/index.js
+++ b/index.js
@@ -1,3 +1,5 @@
+'use strict';
+
 const each = require('lodash.foreach');
 const get = require('lodash.get');