Upgrade examples iris and iris-fitDataset to tfjs 2.6.0 (#367)

Manually tested the two examples to make sure they continue to work.

Author: caisq

iris-fitDataset/package.json

@@ -9,8 +9,8 @@
     "node": ">=8.9.0"
   },
   "dependencies": {
-    "@tensorflow/tfjs": "2.0.0",
-    "@tensorflow/tfjs-vis": "^1.1.0"
+    "@tensorflow/tfjs": "^2.6.0",
+    "@tensorflow/tfjs-vis": "^1.4.3"
   },
   "scripts": {
     "build": "cross-env NODE_ENV=production parcel build index.html --no-minify --public-url ./",