Added missing yarn build postinstall

Author: gtalarico

package.json

@@ -5,7 +5,8 @@
   "scripts": {
     "serve": "vue-cli-service serve --open",
     "build": "vue-cli-service build",
-    "lint": "vue-cli-service lint"
+    "lint": "vue-cli-service lint",
+    "postinstall": "yarn build"
   },
   "dependencies": {
     "axios": "^0.18.0",