Require Symfony ^3.4|^4.0 and add testing for Symfony 4.1 on Travis CI

composer.json

@@ -13,8 +13,8 @@
         "php": "^7.1",
 
         "behat/behat": "^3.4",
-        "symfony/http-kernel": "^3.3|^4.0",
-        "symfony/dotenv": "^3.3|^4.0"
+        "symfony/http-kernel": "^3.4|^4.0",
+        "symfony/dotenv": "^3.4|^4.0"
     },
     "require-dev": {
         "behat/mink": "^1.7",
@@ -22,7 +22,7 @@
         "behat/mink-extension": "^2.2",
         "friends-of-behat/cross-container-extension": "^1.0",
         "friends-of-behat/test-context": "^1.0",
-        "symfony/framework-bundle": "^3.3|^4.0"
+        "symfony/framework-bundle": "^3.4|^4.0"
     },
     "suggest": {
         "behat/mink-browserkit-driver": "^1.3",