Add Symfony 4.3 as a new job in CI

Author: Jean85

.travis.yml

@@ -34,6 +34,8 @@ jobs:
       env: 
       - SYMFONY_VERSION: 3.4.*
       - SYMFONY_DEPRECATIONS_HELPER: disabled
+    - php: 7.3
+      env: SYMFONY_VERSION=4.3.*
     - php: 7.3
       env: SYMFONY_VERSION=4.4.*
     - php: 7.4