remove travis build badges

The k8shim and core repositories still show travis build badges. We have not used travis in more than a year. Those badges part of the top level README should be removed.

We can replace it with the badge generated from the github workflow. As an example for the core:

https://github.com/apache/incubator-yunikorn-core/actions/workflows/main.yml/badge.svg

That brings us inline with the website badge.