Remove Travis

Remove travis now that is has been replaced by Github actions.

.travis.yml

-language: minimal
-
-matrix:
-  include:
-    - name: "7.1"
-      env: PHP_VERSION=7.1
-    - name: "7.2"
-      env: PHP_VERSION=7.2
-    - name: "7.3"
-      env: PHP_VERSION=7.3
-
-services:
-  - docker
-
-cache:
-  directories:
-    - $HOME/.composer/cache
-
-install:
-  - docker version
-  - sudo curl -L "https://github.com/docker/compose/releases/download/1.25.0/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose
-  - docker-compose version
-  - docker-compose build --build-arg PHP_VERSION=${PHP_VERSION}
-  - docker-compose run --rm tests composer install --no-interaction
-
-script:
-  - docker-compose run --rm tests ./vendor/bin/phpunit --coverage-clover ./clover.xml