Allow buck to run when watchman isn't installed

This only runs the watchman command if watchman is on the PATH.

Buck was giving me:

~/gitroot/buck (master)$ bin/buck --help
/home/kchodorow/gitroot/buck/bin/buck_common: line 68: watchman: command not found
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/lib/python2.7/json/__init__.py", line 326, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded