Add PHP 7.3 to test matrix

Author: DanielRuf

.travis.yml

@@ -1,6 +1,8 @@
 language: php
 matrix:
   include:
+  - php: 7.3
+    env: PREFER_LOWEST=""
   - php: 7.2
     env: PREFER_LOWEST=""
   - php: 7.1