Merge branch '4.2' into 4.3

* 4.2: (56 commits)
  Adopt "flex-require" from v1.3
  Update dependencies - v4.2.9.8
  Update dependencies - v3.4.28.7
  Update dependencies - v4.2.9.7
  Update dependencies - v3.4.28.6
  Update dependencies - v4.2.9.6
  Update dependencies - v4.2.9.5
  Update dependencies - v3.4.28.5
  Update dependencies - v4.2.9.4
  Update dependencies - v3.4.28.4
  Update dependencies - v4.2.9.3
  Update dependencies - v3.4.28.3
  Update dependencies - v4.2.9.2
  Update dependencies - v3.4.28.2
  Update dependencies - v4.2.9.1
  Update dependencies - v3.4.28.1
  Update dependencies - v4.2.8.5
  Update dependencies - v3.4.27.5
  Update dependencies - v4.2.8.4
  Update dependencies - v3.4.27.4
  ...

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/http-client": "*",
@@ -29,7 +31,7 @@
         "symfony/web-link": "*",
         "symfony/yaml": "*"
     },
-    "require-dev": {
+    "flex-require-dev": {
         "symfony/debug-pack": "*",
         "symfony/maker-bundle": "^1.0",
         "symfony/profiler-pack": "*",