Kokoro is more responsive than Travis-CI and AppVeyor, probably because we're on the free tier of both those services. We can afford the time to cover more cases with Kokoro.

Thanks to @ehsannas for setting up Kokoro bots, and to prove that the logs can be made public, e.g. see
#466

Last items to switch over fully:

• Cover all VisualStudio cases that AppVeyor covers:
  • VS 2013 Release build
  • VS 2017 Release build
  • VS 2017 Debug build
  • Remove VS 2015 Debug build. (Keep the release build)
• Update badges on the README:
  • Kokoro bot result badges on the README should point to the logs for the latest run.
  • Remove the Travis and AppVeyor badges
  • Turn off the AppVeyor bots
  • Turn off the Travis-CI bots