[Filesystem] restore ability to create broken symlinks
Merge branch '2.6' into 2.7

* 2.6:
  [FrameworkBundle] fix browserkit requirement
  [FrameworkBundle] fix expression-language requirement
  [TwigBridge] fix form requirement
  [Filesystem] restore ability to create broken symlinks
  [FrameworkBundle] fix http-foundation requirement
  [CssSelector] added the license of the Python library we ported to PHP
  [SecurityBundle] avoid unneeded work