Disable Travis bots, and Appveyor Release bots.

Kokoro bots currently cover all Travis bots:
macos-clang-debug
macos-clang-release
ubuntu-clang-debug
ubuntu-clang-release
ubuntu-gcc-debug
ubuntu-gcc-release
ndk-build
android
check-format

Kokoro bots also cover all Windows Release builds:
windows-VS2013-release
windows-VS2015-release
windows-VS2017-release

Due to a compiler issue on the Kokoro Windows VM, we currently do not
run the Debug build on Kokoro.

Therefore, I am disabling all Travis jobs and the appveyor Release jobs.