updated VERSION for 2.3.20
bumped Symfony version to 2.3.21
fix components tests

Since #12006, the `ContainerBuilder` contains the
`addExpressionLanguageProvider()` method which references a class from
the ExpressionLanguage component. By default, the PHPUnit mock API
tries to mock all methods of the class being doubled. Since the
ExpressionLanguage component is not required to run the tests,
creating the mock objects fails when the mock API fails to mock
the `addExpressionLanguageProvider()` method.
[Doc] Use Markdown syntax highlighting
Merge branch 'pull/12053' into 2.3

* pull/12053:
  [Doc] Use Markdown syntax highlighting

Conflicts:
	src/Symfony/Component/Finder/README.md
Merge branch '2.3' into 2.5

* 2.3:
  [Doc] Use Markdown syntax highlighting
  [Finder] tweaked docs
  [Finder] Add info about possibilities offered by SplFileInfo
  fix components tests
  [Intl] FIxed failing test
  [Intl] Generated the data for ICU version 54-rc
  [EventDispatcher] fix doc bloc on EventDispatcherInterface
  [Validator] Update validators.zh_CN.xlf, fix translation error
  bumped Symfony version to 2.3.21
  updated VERSION for 2.3.20
  update CONTRIBUTORS for 2.3.20
  updated CHANGELOG for 2.3.20
  [Intl] Integrated ICU data into Intl component

Conflicts:
	src/Symfony/Component/HttpKernel/Kernel.php
	src/Symfony/Component/Intl/ResourceBundle/LocaleBundle.php