Add ci_mode to disable download progress

[mkg20001]

Otherwise Travis CI complains about log size

Ref kivy/buildozer#823

[AndreMiras]

Very nice, thank you! May I suggest to directly use something from TravisCI?
https://docs.travis-ci.com/user/environment-variables/#default-environment-variables
What I mean is to replace CI_MODE directly by CI so we don't have to bother setting it in .travis.yml since it's already automatically set by Travis. In a same fashion there's a predefined variable called TRAVIS, but then it feels a bit coupled. If you prefer you can also use the predefined CONTINUOUS_INTEGRATION variable rather than CI, I leave that to you

[mkg20001]

@AndreMiras I've switched to the $CI variable. Both in this pr and kivy/buildozer#833

[mkg20001]

I've just noticed: The android SDK installs still continue to spam the logs

Would somebody want to look into that?

[AndreMiras]

Thanks for addressing the comments!
Yes the logs are still showing it because we still need to pass the environment variable from the host to the Docker container. So basically in .travis.yml you can edit the docker run command to add -e CI, so final command should be something like:
docker run -e CI p4a /bin/sh -c "$COMMAND"

[AndreMiras]

Awesome, thanks!