Default test runner doesn't work

[keith]

With the smallest possible project, ios_unit_test fails with this exception:

Traceback (most recent call last):
  File "/Users/ksmiley/.pyenv/versions/2.7.15/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/Users/ksmiley/.pyenv/versions/2.7.15/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "../xctestrunner/file/downloaded/__main__.py", line 312, in <module>
  File "../xctestrunner/file/downloaded/__main__.py", line 306, in main
  File "../xctestrunner/file/downloaded/__main__.py", line 215, in _SimulatorTest
  File "../xctestrunner/file/downloaded/__main__.py", line 170, in _RunSimulatorTest
  File "../xctestrunner/file/downloaded/__main__/xctestrunner/simulator_control/simulator_util.py", line 308, in CreateNewSimulator
  File "../xctestrunner/file/downloaded/__main__/xctestrunner/simulator_control/simulator_util.py", line 506, in GetLastSupportedSimOsVersion
  File "../xctestrunner/file/downloaded/__main__/xctestrunner/simulator_control/simulator_util.py", line 464, in GetSupportedSimOsVersions
KeyError: 'availability'

With this sample project: test_xctestrunner.zip you can reproduce with bazel test test --host_force_python=PY2 (the forcing python isn't related it's just suppressing another issue)