Merge branch '3.3' into 3.4

* 3.3:
  Adopt "flex-require" from v1.3

Author: nicolas-grekas

composer.json

@@ -7,12 +7,14 @@
         "php": "^7.0.8",
         "ext-ctype": "*",
         "ext-iconv": "*",
+        "symfony/flex": "^1.3.1"
+    },
+    "flex-require": {
         "sensio/framework-extra-bundle": "^5.1",
         "symfony/asset": "*",
         "symfony/console": "*",
         "symfony/dotenv": "*",
         "symfony/expression-language": "*",
-        "symfony/flex": "^1.1",
         "symfony/form": "*",
         "symfony/framework-bundle": "*",
         "symfony/monolog-bundle": "^3.1",
@@ -27,7 +29,7 @@
         "symfony/web-link": "*",
         "symfony/yaml": "*"
     },
-    "require-dev": {
+    "flex-require-dev": {
         "symfony/debug-pack": "*",
         "symfony/maker-bundle": "^1.0",
         "symfony/profiler-pack": "*",