Remove remaining references to Travis CI

Commit 1edc1264 replaced Travis CI with GitHub Actions. Remove the
remaining references to Travis CI.

(cherry picked from commit 4ce4c737)

.gitattributes

@@ -7,5 +7,4 @@
 .editorconfig export-ignore
 .gitattributes export-ignore
 .gitignore export-ignore
-.travis.yml export-ignore
 

TESTING.md

@@ -65,7 +65,6 @@ the old version installed by composer
 ./vendor/bin/phpunit -c ./phpunit.xml
 ```
 
-All the tests are run by our *continuous integration* platform,
-[travis](https://travis-ci.org/simplesamlphp/simplesamlphp). If you are
-submitting a pull request, Travis will run your tests and notify whether
-your code builds or not according to them.
+All the tests are run by our *continuous integration* platform using GitHub
+Actions. If you are submitting a pull request, GitHub Actions will run your
+tests and notify whether your code builds or not according to them.

bin/build-release.sh

@@ -61,7 +61,6 @@ rm "$TARGET/www/assets/js/stylesheet.js"*
 rm "$TARGET/.editorconfig"
 rm "$TARGET/.gitattributes"
 rm "$TARGET/.php_cs.dist"
-rm "$TARGET/.travis.yml"
 rm "$TARGET/codecov.yml"
 rm "$TARGET/psalm.xml"
 rm "$TARGET"/{,modules}/.gitignore