Merge branch '3.1'

* 3.1: (22 commits)
  [travis] Fix deps=low/high builds
  [Form] Fix depreciation triggers
  fixed CS
  skip test with current phpunit bridge
  Fix for #19183 to add support for new PHP MongoDB extension in sessions.
  [Console] Fix for block() padding formatting after #19189
  [Security][Guard] check if session exist before using it
  bumped Symfony version to 3.1.3
  updated VERSION for 3.1.2
  updated CHANGELOG for 3.1.2
  bumped Symfony version to 3.0.9
  updated VERSION for 3.0.8
  updated CHANGELOG for 3.0.8
  bumped Symfony version to 2.8.9
  updated VERSION for 2.8.8
  updated CHANGELOG for 2.8.8
  bumped Symfony version to 2.7.16
  updated VERSION for 2.7.15
  update CONTRIBUTORS for 2.7.15
  updated CHANGELOG for 2.7.15
  ...

Conflicts:
	src/Symfony/Component/HttpKernel/Kernel.php

Author: nicolas-grekas

Form/ChoiceList/DoctrineChoiceLoader.php

@@ -69,7 +69,7 @@ public function __construct($manager, $class, $idReader = null, $objectLoader =
     {
         // BC to be removed and replace with type hints in 4.0
         if ($manager instanceof ChoiceListFactoryInterface) {
-            @trigger_error(sprintf('Passing a ChoiceListFactoryInterface to %s is deprecated since version 3.1 and will no longer be supported in 4.0. You should either call "%s::loadChoiceList" or override it to return a ChoiceListInterface.', __CLASS__, __CLASS__));
+            @trigger_error(sprintf('Passing a ChoiceListFactoryInterface to %s is deprecated since version 3.1 and will no longer be supported in 4.0. You should either call "%s::loadChoiceList" or override it to return a ChoiceListInterface.', __CLASS__, __CLASS__), E_USER_DEPRECATED);
 
             // Provide a BC layer since $factory has changed
             // form first to last argument as of 3.1