Switched serial from 'yes' to '1' for Ansible 1.8.1.

Prior to change I was seeing the following error with Ansible 1.8.1:

GATHERING FACTS *************************************************************** 
ok: [host-02]
ok: [host-03]
ok: [host-01]
Traceback (most recent call last):
  File "/usr/local/bin/ansible-playbook", line 324, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/usr/local/bin/ansible-playbook", line 264, in main
    pb.run()
  File "/Library/Python/2.7/site-packages/ansible/playbook/__init__.py", line 347, in run
    if not self._run_play(play):
  File "/Library/Python/2.7/site-packages/ansible/playbook/__init__.py", line 692, in _run_play
    serial = int(play.serial)
ValueError: invalid literal for int() with base 10: 'True'