tests: Added Symfony 4 to test matrix

Author: tarlepp

.travis.yml

@@ -10,6 +10,8 @@ matrix:
       env: SYMFONY_VERSION=3.1.*
     - php: 7.0
       env: SYMFONY_VERSION=3.2.*
+    - php: 7.1
+      env: SYMFONY_VERSION=4.0.*
 
 before_install:
   - >