Add explicit framework version for iris example.

Author: nadiaya

sagemaker-python-sdk/tensorflow_iris_dnn_classifier_using_estimators/tensorflow_iris_dnn_classifier_using_estimators.ipynb

@@ -300,6 +300,7 @@
     "\n",
     "iris_estimator = TensorFlow(entry_point='iris_dnn_classifier.py',\n",
     "                            role=role,\n",
+    "                            framework_version='1.8',\n",
     "                            output_path=model_artifacts_location,\n",
     "                            code_location=custom_code_upload_location,\n",
     "                            train_instance_count=1,\n",