Merge branch '2.7' into 2.8

* 2.7:
  [HttpKernel] Fix lowest dep
  [Security] fix check for empty usernames
  [Form] updated exception message of ButtonBuilder::setRequestHandler()
  [travis] Fix deps=high jobs
  Fix typo 'assets.package' => 'assets.packages' in UPGRADE-2.7
  [Serializer] Simplify AbstractNormalizer::prepareForDenormalization()
  [HttpFoundation] [PSR-7] Allow to use resources as content body and to return resources from string content
  [DependencyInjection] Remove unused code in XmlFileLoader
  [HttpFoundation] Behaviour change in PHP7 for substr
  bumped Symfony version to 2.3.32
  updated VERSION for 2.3.31
  update CONTRIBUTORS for 2.3.31
  updated CHANGELOG for 2.3.31

Conflicts:
	src/Symfony/Bridge/Twig/composer.json
	src/Symfony/Bundle/FrameworkBundle/composer.json
	src/Symfony/Component/HttpKernel/composer.json

Author: nicolas-grekas

composer.json

@@ -18,8 +18,8 @@
     "require": {
         "php": ">=5.3.9",
         "symfony/asset": "~2.7|~3.0.0",
-        "symfony/dependency-injection" : "~2.8",
-        "symfony/config" : "~2.4",
+        "symfony/dependency-injection": "~2.8",
+        "symfony/config": "~2.4",
         "symfony/event-dispatcher": "~2.8|~3.0.0",
         "symfony/http-foundation": "~2.4.9|~2.5,>=2.5.4|~3.0.0",
         "symfony/http-kernel": "~2.7|~3.0.0",