Add PHP 7.3 to test matrix

Author: mbabker

.travis.yml

@@ -25,6 +25,7 @@ matrix:
       # This empty flag removes the prefer-stable switch to cause dev dependencies to be installed
       env: COMPOSER_FLAGS=""
     - php: 7.2
+    - php: 7.3
     - php: nightly
   allow_failures:
     - php: nightly