Specified Node JS version to fix a Travis deployment error

Author: jongpie

.travis.yml

@@ -1,5 +1,8 @@
 language: node_js
 
+node_js:
+    - "7"
+
 script:
     - npm install -g jsforce-metadata-tools
     - jsforce-deploy --dry-run -u $DEPLOYMENT_USERNAME -p $DEPLOYMENT_PASSWORD$DEPLOYMENT_TOKEN -D $TRAVIS_BUILD_DIR/src -l $DEPLOYMENT_LOGIN_URL --rollbackOnError true --testLevel $DEPLOYMENT_TEST_LEVEL --pollTimeout $POLL_TIMEOUT