doc: Drop mentions of Travis CI as it is no longer used

README.md

@@ -56,7 +56,8 @@ There are also [regression and integration tests](/test), written
 in Python, that are run automatically on the build server.
 These tests can be run (if the [test dependencies](/test) are installed) with: `test/functional/test_runner.py`
 
-The Travis CI system makes sure that every pull request is built for Windows, Linux, and macOS, and that unit/sanity tests are run automatically.
+The CI (Continuous Integration) systems make sure that every pull request is built for Windows, Linux, and macOS,
+and that unit/sanity tests are run automatically.
 
 ### Manual Quality Assurance (QA) Testing