feature #800 Updated the app to Symfony 4.1 beta1 (javiereguiluz)

This PR was merged into the master branch.

Discussion
----------

Updated the app to Symfony 4.1 beta1

Commits
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ccf56b9 Updated the app to Symfony 4.1 beta1

composer.json

@@ -3,6 +3,7 @@
     "license": "MIT",
     "type": "project",
     "description": "Symfony Demo Application",
+    "minimum-stability": "beta",
     "require": {
         "php": "^7.1.3",
         "ext-pdo_sqlite": "*",
@@ -14,32 +15,32 @@
         "ezyang/htmlpurifier": "^4.9",
         "sensio/framework-extra-bundle": "^5.0",
         "sensiolabs/security-checker": "^4.1",
-        "symfony/asset": "^4.0",
-        "symfony/expression-language": "^4.0",
+        "symfony/asset": "^4.1@beta",
+        "symfony/expression-language": "^4.1@beta",
         "symfony/flex": "^1.0",
-        "symfony/form": "^4.0",
-        "symfony/framework-bundle": "^4.0",
+        "symfony/form": "^4.1@beta",
+        "symfony/framework-bundle": "^4.1@beta",
         "symfony/monolog-bundle": "^3.1",
-        "symfony/security-bundle": "^4.0",
+        "symfony/security-bundle": "^4.1@beta",
         "symfony/swiftmailer-bundle": "^3.1",
-        "symfony/translation": "^4.0",
-        "symfony/twig-bundle": "^4.0",
-        "symfony/validator": "^4.0",
-        "symfony/yaml": "^4.0",
+        "symfony/translation": "^4.1@beta",
+        "symfony/twig-bundle": "^4.1@beta",
+        "symfony/validator": "^4.1@beta",
+        "symfony/yaml": "^4.1@beta",
         "twig/extensions": "^1.5",
         "white-october/pagerfanta-bundle": "^1.1"
     },
     "require-dev": {
         "dama/doctrine-test-bundle": "^5.0",
         "friendsofphp/php-cs-fixer": "^2.7",
-        "symfony/browser-kit": "^4.0",
-        "symfony/css-selector": "^4.0",
-        "symfony/debug-bundle": "^4.0",
-        "symfony/dotenv": "^4.0",
-        "symfony/phpunit-bridge": "^4.0",
-        "symfony/stopwatch": "^4.0",
-        "symfony/web-profiler-bundle": "^4.0",
-        "symfony/web-server-bundle": "^4.0"
+        "symfony/browser-kit": "^4.1@beta",
+        "symfony/css-selector": "^4.1@beta",
+        "symfony/debug-bundle": "^4.1@beta",
+        "symfony/dotenv": "^4.1@beta",
+        "symfony/phpunit-bridge": "^4.1@beta",
+        "symfony/stopwatch": "^4.1@beta",
+        "symfony/web-profiler-bundle": "^4.1@beta",
+        "symfony/web-server-bundle": "^4.1@beta"
     },
     "config": {
         "platform": {