Merge branch '4.1' into 4.2

* 4.1: (45 commits)
  Adopt "flex-require" from v1.3
  Update dependencies - v3.4.28.7
  Update dependencies - v3.4.28.6
  Update dependencies - v3.4.28.5
  Update dependencies - v3.4.28.4
  Update dependencies - v3.4.28.3
  Update dependencies - v3.4.28.2
  Update dependencies - v3.4.28.1
  Update dependencies - v3.4.27.5
  Update dependencies - v3.4.27.4
  Update dependencies - v3.4.27.3
  Update dependencies - v3.4.27.2
  Update dependencies - v3.4.27.1
  Update dependencies - v3.4.26.2
  Update dependencies - v3.4.26.1
  Update dependencies - v3.4.24.2
  Update dependencies - v3.4.24.1
  Update dependencies - v3.4.23.3
  Update dependencies - v3.4.23.2
  Update dependencies - v3.4.23.1
  ...

Author: nicolas-grekas

composer.json

@@ -7,12 +7,14 @@
         "php": "^7.1.3",
         "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": "*",