Add module markupsafe to installation instruction

Without 'pip install markupsafe' on Mac OS X 10.9.2 ansible gives the following error:

$ ansible
Traceback (most recent call last):
  File "/Users/simonfi/Documents/ansible/bin/ansible", line 25, in <module>
    from ansible.runner import Runner
  File "/Users/simonfi/Documents/ansible/lib/ansible/runner/__init__.py", line 32, in <module>
    import jinja2
  File "/Library/Python/2.7/site-packages/jinja2/__init__.py", line 33, in <module>
    from jinja2.environment import Environment, Template
  File "/Library/Python/2.7/site-packages/jinja2/environment.py", line 13, in <module>
    from jinja2 import nodes
  File "/Library/Python/2.7/site-packages/jinja2/nodes.py", line 18, in <module>
    from jinja2.utils import Markup
  File "/Library/Python/2.7/site-packages/jinja2/utils.py", line 520, in <module>
    from markupsafe import Markup, escape, soft_unicode
ImportError: No module named markupsafe