[fix] boolean capitalization

Discovered when running `le monitor` on SmartOS on Joyent Cloud.

This fix resolves where the current version causes error:
```py
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/opt/local/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/opt/local/bin/le", line 483, in run
    msg = self.msg_queue.get(true)
NameError: global name 'true' is not defined
```

Thanks for this lib!