message: Add missing python dependency.
diff: @@ -168,6 +168,7 @@ If bazel is not installed on your system, install it now by following
  To build TensorFlow, you must install the following packages:
  * six
  +* mock
  * numpy, which is a numerical processing package that TensorFlow requires.
  * wheel, which enables you to manage Python compressed packages in the wheel
  (.whl) format.
  @@ -179,7 +180,7 @@ If you follow these instructions, you will not need to disable SIP.
  After installing pip, invoke the following commands:
  -<pre> $ <b>sudo pip install six numpy wheel</b> </pre>
  +<pre> $ <b>sudo pip install six numpy wheel mock</b> </pre>
  Note: These are just the minimum requirements to _build_ tensorflow. Installing
  the pip package will download additional packages required to _run_ it. If you
  