Change get-pip url

The path for get-pip.py script in version 3.5 has been changed
with this commit [1].

[1] 2360f025eb

Change-Id: Ie13a6597c23c0a376f9feba2aed664e1129c5b60

playbooks/roles/pip3/tasks/xenial.yaml

@@ -1,6 +1,6 @@
 # https://github.com/pypa/get-pip/issues/83
 - name: Download get-pip.py
-  command: wget https://bootstrap.pypa.io/3.5/get-pip.py
+  command: wget https://bootstrap.pypa.io/pip/3.5/get-pip.py
   args:
     chdir: /var/lib
     creates: /var/lib/get-pip.py