Replace builds with buildCommand in vercel.json

vercel.json

@@ -6,15 +6,7 @@
   "github": {
     "silent": true
   },
-  "builds": [
-    {
-      "src": "package.json",
-      "use": "@now/static-build",
-      "config": {
-        "distDir": "__sapper__/export"
-      }
-    }
-  ],
+  "buildCommand": "yarn build",
   "routes": [
     {
       "src": "^/service-worker\\.js$",