Use absolute rather than relative path for config.yml

The relative path is relative to the files/ directory and so the include
breaks if that directory is deleted.

https://groups.google.com/forum/#!topic/ansible-project/MzSZLYhx5AY

Therefore seems more robust to switch to an absolute path making use of
`playbook_dir`.

main.yml

@@ -8,7 +8,7 @@
   pre_tasks:
     - include_vars: "{{ item }}"
       with_fileglob:
-        - ../config.yml
+        - "{{ playbook_dir }}/config.yml"
       tags: ['always']
 
   roles: