Merge branch '2.8' into 3.0

* 2.8:
  Create PULL_REQUEST_TEMPLATE.md
  Remove duplicate validation in RedirectResponse
  [Yaml] fix default timezone to be UTC
  [DependencyInjection] fix dumped YAML string
  [Config] Add suggest to composer.json for symfony/yaml

Conflicts:
	src/Symfony/Component/DependencyInjection/Dumper/YamlDumper.php

Author: nicolas-grekas

composer.json

@@ -19,6 +19,9 @@
         "php": ">=5.5.9",
         "symfony/filesystem": "~2.8|~3.0"
     },
+    "suggest": {
+        "symfony/yaml": "To use the yaml reference dumper"
+    },
     "autoload": {
         "psr-4": { "Symfony\\Component\\Config\\": "" },
         "exclude-from-classmap": [