Pin get-pip.py to 3.2

As get-pip.py evolves based on pip 10, while we're still using
pip 9, changes in the way it can be used are causing problems.

For example, the ability to use --download is no longer there.

As such, let's pin to a known good version and leave it at that
until we no longer need to use this script. Version 3.2 maps to
pip 7.1.2 which fulfills our needs.

Change-Id: Ic9210c3389776385286f6215e6673d75fa2d01d9

doc/source/contributor/extending_yaql.rst

@@ -121,7 +121,7 @@ For more information about YAQL, read the `official YAQL documentation <http://y
 
 .. code-block:: bash
 
-    $ curl https://bootstrap.pypa.io/get-pip.py | python
+    $ curl https://bootstrap.pypa.io/3.2/get-pip.py | python
     $ pip install --upgrade setuptools
     $ cd -
 

tools/docker/Dockerfile

@@ -21,7 +21,7 @@ RUN apt-get -qq update && \
       gcc \
       libuv1 \
       libuv1-dev && \
-      curl -f -o /tmp/get-pip.py https://bootstrap.pypa.io/get-pip.py && \
+      curl -f -o /tmp/get-pip.py https://bootstrap.pypa.io/3.2/get-pip.py && \
       python /tmp/get-pip.py && rm /tmp/get-pip.py
 
 RUN pip install pymysql psycopg2 py_mini_racer