Update blog_estimators_dataset.py

Added assert for TensorFlow r1.4 since tf.data is required.

samples/outreach/blogs/blog_estimators_dataset.py

@@ -25,7 +25,7 @@ import tensorflow as tf
 # Check that we have correct TensorFlow version installed
 tf_version = tf.__version__
 print("TensorFlow version: {}".format(tf_version))
-assert "1.3" <= tf_version, "TensorFlow r1.3 or later is needed"
+assert "1.4" <= tf_version, "TensorFlow r1.4 or later is needed"
 
 # Windows users: You only need to change PATH, rest is platform independent
 PATH = "/tmp/tf_dataset_and_estimator_apis"