Changes to homu config encounter salt error

[jdm]

The "salt from upgrade" CI job fails:

----------

          ID: homu

    Function: pip.installed

      Result: False

     Comment: An exception occurred in this state: Traceback (most recent call last):

                File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1933, in call

                  **cdata['kwargs'])

                File "/usr/lib/python2.7/dist-packages/salt/loader.py", line 1951, in wrapper

                  return f(*args, **kwargs)

                File "/usr/lib/python2.7/dist-packages/salt/states/pip_state.py", line 812, in installed

                  **kwargs)

                File "/usr/lib/python2.7/dist-packages/salt/states/pip_state.py", line 314, in _check_if_installed

                  for version in reversed(available_versions):

              TypeError: argument to reversed() must be a sequence

     Started: 22:51:27.656454

    Duration: 1829.83 ms

     Changes:   

[jdm]

This is preventing merging #1007 right now.

[jdm]

Attempting to investigate in https://github.com/servo/saltfs/compare/master...jdm-patch-72?quick_pull=1.