Command line deployment fails for Greeter

* Populus Version: 1.6.9
* OS: osx

### What was wrong?

Hi, I have changed "populous.json" as explained in the document
for creating a local chain, started a local chain, waited for blocks
being mined, and tried

$ populus deploy Greeter -c bch

where bch is the name of my local chain.
Then I got an error whose last lines go as follows:

-----
File "/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/lib/python2.7/collections.py", line 566, in update
for elem in iterable:
File "/usr/local/lib/python2.7/site-packages/populus/utils/config.py", line 106, in
for backend_name, config
File "/usr/local/lib/python2.7/site-packages/populus/config/base.py", line 137, in __getitem__
raise KeyError("Key '{0}' not found in {1}".format(key, self._wrapped))
KeyError: "Key 'priority' not found in {}"
-----

I have used pip to upgrade all dependencies.