Fix stevedore on Appveyor

[nuest]

Stevedore does not work on appveyor, so the vignettes and some tests do not work:

Rscript -e "stevedore::docker_available(); stevedore::docker_client()$api_version()"
[1] FALSE
Error in py_module_import(module, convert = convert) : 
  ImportError: cannot import name 'NpipeAdapter'
Detailed traceback: 
  File "c:/RLibrary/stevedore/py\httppipe.py", line 9, in <module>
    from docker.transport import NpipeAdapter as HttpAdapter
Calls: <Anonymous> ... httppipe_prepare -> <Anonymous> -> py_module_import
Execution halted
Command exited with code 1
7z a failure.zip *.Rcheck\*

See full build output at https://ci.appveyor.com/project/nuest/containerit/builds/26171500

[vsoch]

In case you want to try something different, CircleCI just announced support for Windows. https://devclass.com/2019/08/08/circleci-finally-open-up-to-windows-devs/

[nuest]

And they also support Docker: https://circleci.com/docs/2.0/building-docker-images/