Add python-yaml to the dependencies

installing ensemble from the ppa doesn't install python-yaml

daker@ubuntu:~$ ensemble bootstrap
Traceback (most recent call last):
  File "/usr/bin/ensemble", line 4, in <module>
    from ensemble.control import main
  File "/usr/lib/pymodules/python2.7/ensemble/control/__init__.py", line 7, in <module>
    from ensemble.environment.config import EnvironmentsConfig
  File "/usr/lib/pymodules/python2.7/ensemble/environment/config.py", line 2, in <module>
    import yaml
ImportError: No module named yaml