'gordon' with no args raises error

[Brian Candler]

Trivial problem:

$ gordon
Traceback (most recent call last):
  File "/home/ubuntu/aws/venv/bin/gordon", line 11, in <module>
    sys.exit(main())
  File "/home/ubuntu/aws/venv/lib/python3.5/site-packages/gordon/bin.py", line 108, in main
    obj = options.cls(path=path, stdin=stdin, **vars(options))
AttributeError: 'Namespace' object has no attribute 'cls'

This is with Python 3.5.2 under Ubuntu 16.04

gordon build and gordon apply are fine though