Switch to Yarn for CI build. (#290)

Author: jmdobry

vision/samples/package.json

@@ -5,7 +5,7 @@
   "license": "Apache Version 2.0",
   "author": "Google Inc.",
   "scripts": {
-    "test": "cd ..; npm run st -- vision/system-test/*.test.js"
+    "test": "cd ..; npm run st -- --verbose vision/system-test/*.test.js"
   },
   "dependencies": {
     "@google-cloud/vision": "0.7.0",