fix node 6 test (#560)

Author: strausr

.travis.yml

@@ -4,7 +4,7 @@ nvm:
 matrix:
   include:
   - node_js: "6"
-    script: npm run test-with-temp-cloud
+    script: npm install [email protected] && npm run test-with-temp-cloud
   - node_js: "8"
     script: npm run test-with-temp-cloud
   - node_js: "10"