Remove references to obsolete Travis config (#426)

* Delete obsolete .travis.yml

Prefer alternative CI.

* Remove reference to obsolete Travis build

Author: mtrea

.travis.yml

@@ -1,6 +1,5 @@
 # Bazel Web Testing Rules
 
-[![Build Status](https://travis-ci.org/bazelbuild/rules_webtesting.svg?branch=master)](https://travis-ci.org/bazelbuild/rules_webtesting)
 [![Build status](https://badge.buildkite.com/d9c3974f925876394ca9d3e00670c0950b6f7ebf325412def7.svg?branch=master)](https://buildkite.com/bazel/rules-webtesting-saucelabs)
 
 Bazel rules and supporting code to allow testing against a browser with