Update https://docs.bazel.build/versions/master/skylark/deploying.html#testing for buildkite

[hlopko]

Hello engprod :)

I noticed we still advice users to use Travis on https://docs.bazel.build/versions/master/skylark/deploying.html#testing, could you maybe update the documentation to mention Buildkite and the Bazel CI?

Thanks! :)

[hlopko]

cc @fweikert @philwo