Move out of Travis container-based infrastructure and added PHP 7.3

.travis.yml

@@ -7,6 +7,7 @@ branches:
 php:
   - 7.1
   - 7.2
+  - 7.3
 
 before_script:
   - composer self-update
@@ -19,5 +20,3 @@ script:
 after_script:
   - wget https://scrutinizer-ci.com/ocular.phar
   - php ocular.phar code-coverage:upload --format=php-clover build/clover.xml
-
-sudo: false