ci(travis): remove .travis.yml config file

If we're switching to GitHub Actions, there's no need to keep the
Travis-CI config file around.

.travis.yml

@@ -1,24 +0,0 @@
-dist: trusty
-language: php
-dist: trusty
-
-php:
-  - '7.4'
-  - '7.3'
-  - '7.2'
-  - '7.1'
-
-before_install:
-  - composer update
-
-script:
-  - composer validate
-  - vendor/bin/phpunit --version
-  - vendor/bin/phpunit --configuration tests/phpunit.xml
-
-notifications:
-  email:
-    recipients:
-      - will.knauss@gmail.com
-    on_success: never
-    on_failure: always