Temporarily use non-default branches of two Ansible roles

This change is being made for testing purposes, and will be reverted
after the following PRs are merged:
- cisagov/ansible-role-pip#50
- cisagov/ansible-role-python#46

packer/ansible/requirements.yml

@@ -64,8 +64,10 @@
   name: persist_journald
 - src: https://github.com/cisagov/ansible-role-pip
   name: pip
+  version: improvement/control-installation-of-pip2-via-role-var
 - src: https://github.com/cisagov/ansible-role-python
   name: python
+  version: improvement/control-installation-of-python2-via-role-var
 - src: https://github.com/cisagov/ansible-role-remove-python2
   name: remove_python2
 - src: https://github.com/cisagov/ansible-role-upgrade