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

Tests/Fragment/EsiFragmentRendererTest.php

@@ -28,6 +28,7 @@ public function testRenderFallbackToInlineStrategyIfEsiNotSupported()
 
     /**
      * @group legacy
+     * @requires function Symfony\Bridge\PhpUnit\ErrorAssert::assertDeprecationsAreTriggered
      */
     public function testRenderFallbackWithObjectAttributesIsDeprecated()
     {