Change checkpoint suffix to "ckpt" (#3470)

Tensorflow doesn't prescribe any particular file suffix for checkpoint files, but they 
are commonly referred to as "ckpt" as a shorthand for "checkpoint".  However ours 
is somewhat confusingly "cptk".  This change simply changes our checkpoint suffix 
to "ckpt".

com.unity.ml-agents/CHANGELOG.md

  - A tutorial on adding custom SideChannels was added (#3391)
  - The stepping logic for the Agent and the Academy has been simplified (#3448)
  - Update Barracuda to 0.6.0-preview
+ - The checkpoint file suffix was changed from `.cptk` to `.ckpt` (#3470)
 
 ### Bugfixes
 - Fixed an issue which caused self-play training sessions to consume a lot of memory. (#3451)

ml-agents/mlagents/trainers/tf_policy.py

         :return:
         """
         with self.graph.as_default():
-            last_checkpoint = self.model_path + "/model-" + str(steps) + ".cptk"
+            last_checkpoint = self.model_path + "/model-" + str(steps) + ".ckpt"
             self.saver.save(self.sess, last_checkpoint)
             tf.train.write_graph(
                 self.graph, self.model_path, "raw_graph_def.pb", as_text=False