added travis ci cache setting

.travis.yml

@@ -1,5 +1,6 @@
 sudo: false
 language: node_js
+
 node_js:
   - "0.10"
   - "0.12"
@@ -7,6 +8,10 @@ node_js:
   - "4"
   - "stable"
 
+cache:
+  directories:
+    - node_modules
+
 script: "npm run travis"
 
 after_success: