updating travis cache directive

Author: Parth Shah

.travis.yml

@@ -1,8 +1,13 @@
 language: node_js
 node_js:
 - 5.5.0
-cache: yarn
+cache:
+  yarn: true
+  directories:
+    - node_modules
+
 script:
+- if [ -n "$CLEAN_CACHE" ]; then echo 'Cleaning cache'; yarn cache clean; fi
 - npm run lint && npm test && npm run build
 sudo: false
 addons: