Add Symfony version 3.4.x (LTS) to Travis CI testing scenarios

Author: picodexter

.travis.yml

@@ -23,6 +23,8 @@ matrix:
       env: SYMFONY_VERSION="2.7.*"
     - php: 5.6
       env: SYMFONY_VERSION="2.8.*"
+    - php: 5.6
+      env: SYMFONY_VERSION="3.4.*"
     # Test against dev versions
     - php: 5.6
       env: DEPENDENCIES=dev