Use PHP 7.3 for symfony 5

Travis does not have PHP 7.2.5, so switching to PHP 7.3 for testing.

Author: DariusIII

.travis.yml

@@ -71,7 +71,8 @@ matrix:
       env: [SF_EVT_DISPATCHER_VERSION="^3.3", SF_OPT_RESOLVER_VERSION="^3.3"]
     - php: 7.2
       env: [SF_EVT_DISPATCHER_VERSION="^4.0", SF_OPT_RESOLVER_VERSION="^4.0"]
-    - php: 7.2
+      
+    - php: 7.3
       env: [SF_EVT_DISPATCHER_VERSION="^5.0", SF_OPT_RESOLVER_VERSION="^5.0"]
 
     - php: hhvm