docs: add travis badge
skaffold
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+[![Build Status](https://travis-ci.com/GoogleCloudPlatform/skaffold.svg?token=NyoV8n1D3L8EzmetKFNB&branch=master)](https://travis-ci.com/GoogleCloudPlatform/skaffold)
+
A suite of tools that makes the onboarding of existing applications to Kubernetes Engine simple and repeatable.
