Update to use Tensorflow 1.15

- Tensorflow 2 is out
- Fixes https://github.com/MicrosoftDocs/azure-docs/issues/41095

Author: anthonychu

articles/azure-functions/functions-machine-learning-tensorflow.md

@@ -175,7 +175,7 @@ Confirm that *classify* now contains a file named *predict.py*.
 The helper library has some dependencies that need to be installed. Open *start/requirements.txt* in your editor and add the following dependencies to the file.
 
 ```txt
-tensorflow
+tensorflow==1.15
 Pillow
 requests
 ```