Fix numpy version to pre-1.19.0

Fix numpy to pre-1.19.0 because of breaking ABI change in numpy 1.19.0 (numpy/numpy#15355)

See tensorflow/tensorflow#40688.

Author: adk9

site/en/install/source.md

@@ -37,7 +37,7 @@ Install the TensorFlow *pip* package dependencies (if using a virtual environmen
 omit the `--user` argument):
 
 <pre class="prettyprint lang-bsh">
-<code class="devsite-terminal">pip install -U --user pip six numpy wheel setuptools mock 'future>=0.17.1'</code>
+<code class="devsite-terminal">pip install -U --user pip six numpy<1.19.0 wheel setuptools mock 'future>=0.17.1'</code>
 <code class="devsite-terminal">pip install -U --user keras_applications --no-deps</code>
 <code class="devsite-terminal">pip install -U --user keras_preprocessing --no-deps</code>
 </pre>