updated node version in travis.yml

Author: yougotwill

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - 6
+  - 7
 script:
   - npm run lint && npm run test
   - 'if [[ ${TRAVIS_PULL_REQUEST_BRANCH:-$TRAVIS_BRANCH} = "master" ]]; then npm install -g grunt npm@5.2 && grunt pre-build; fi'