Refactored the CssSelector to remove the circular object graph

This allows the translator and its extensions to be garbage collected
based on the refcount rather than requiring the garbage collector run,
making it much more likely to happen at the end of the
CssSelector::toXPath call.
Merge branch '2.3' into 2.4

* 2.3:
  Fixed failed config schema loads due to libxml_disable_entity_loader usage.
  enabled PHP 5.6 for tests
  [Process] Fix ExecutableFinder with open basedir
  Refactored the CssSelector to remove the circular object graph
  Fix mocks to support >=5.5.14 and >=5.4.30
  [ClassLoader] fixed PHP warning on PHP 5.3
  [Components][Serializer] optional constructor arguments can be omitted during the denormalization process
  [DomCrawler] properly handle buttons with single and double quotes inside the name attribute

Conflicts:
	src/Symfony/Component/Serializer/Tests/Normalizer/GetSetMethodNormalizerTest.php