Merge branch '2.8' into 3.0

* 2.8:
  [travis] Fix deps=low/high builds
  fixed CS
  skip test with current phpunit bridge
  Fix for symfony#19183 to add support for new PHP MongoDB extension in sessions.
  [Console] Fix for block() padding formatting after symfony#19189
  [Security][Guard] check if session exist before using it
  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
  Fix some lowest deps
  Fixed typos in the expectedException annotations

Conflicts:
	CHANGELOG-2.7.md
	CHANGELOG-3.0.md
	src/Symfony/Bundle/FrameworkBundle/composer.json
	src/Symfony/Component/HttpFoundation/Tests/Session/Storage/Handler/MongoDbSessionHandlerTest.php
	src/Symfony/Component/HttpKernel/Kernel.php
	src/Symfony/Component/HttpKernel/composer.json
	src/Symfony/Component/Yaml/Tests/ParserTest.php

Author: nicolas-grekas

.github/travis.php

@@ -46,8 +46,8 @@
     $versions = file_get_contents('https://packagist.org/packages/'.$package->name.'.json');
     $versions = json_decode($versions);
 
-    foreach ($versions->package->versions as $version => $package) {
-        $packages[$package->name] += array($version => $package);
+    foreach ($versions->package->versions as $v => $package) {
+        $packages[$package->name] += array($v => $package);
     }
 }