Add tests runs on PHP 7.3 in Travis

.travis.yml

@@ -11,6 +11,8 @@ matrix:
       env: WITH_PHPUNIT=true WITH_COVERAGE=true
     - php: 7.2
       env: WITH_PHPUNIT=true
+    - php: 7.3
+      env: WITH_PHPUNIT=true
 
 cache:
   directories: