Replace Travis CI badge with GitHub Actions

Travis CI support was dropped in 8f80fd9.
Update the README badge to link to the GitHub Actions test workflow instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yahonda

README.md

@@ -1,4 +1,4 @@
-[![Build Status](https://travis-ci.com/rsim/ruby-plsql.svg?branch=master)](https://travis-ci.com/rsim/ruby-plsql)
+[![Test](https://github.com/rsim/ruby-plsql/actions/workflows/test.yml/badge.svg)](https://github.com/rsim/ruby-plsql/actions/workflows/test.yml)
 
 ruby-plsql
 ==========