message: make sure CI fails when we do not recognize the platform
diff: @@ -48,4 +48,7 @@ elif [ "${TRAVIS_OS_NAME:-}" == osx ]; then
  elif [ "${CI_WINDOWS:-}" == True ]; then
  MIRI_TEST_TARGET=x86_64-unknown-linux-gnu run_tests
  MIRI_TEST_TARGET=x86_64-apple-darwin run_tests
  +else
  + echo "FATAL: unknown CI platform"
  + exit 1
  fi
  