Fix AMI builds by pinning pip to 18.1

As of pip 19.0, it started printing warnings for Python 2.7, which
are treated as errors by Jenkins. I looked briefly in to ignoring
or silencing those warnings (see pypa/pip#6147 (comment)),
but couldn't configure Jenkins correctly. Thus, I'm going with the
downgrade, as was done for #bees in project-icp/bee-pollinator-app#447.

deployment/packer/template.js

@@ -112,7 +112,7 @@
                 "sleep 5",
                 "sudo apt-get update -qq",
                 "sudo apt-get install python-pip python-dev -y",
-                "sudo pip install --upgrade pip",
+                "sudo pip install --upgrade pip==18.1",
                 "sudo pip install ansible==2.4.4.0",
                 "sudo /bin/sh -c 'echo {{user `branch`}} {{user `description`}} > /srv/version.txt'"
             ]