Merge branch '2.7'

* 2.7:
  Replaced a "note" by a "seealso"
  Imrpoved the wording of the note about using using legacy PHP 5.3 version
  Reworded the reasons why you should use Composer-based installation
  Wrapped a line to follow doc standards
  Display the same version number on Linux and Windows to avoid confusions
  Add an explicit command to better explain that we recommend to move symfony.phar to projects directory
  Reworded a confusing phrase
  Added a note about verifying Symfony digital signatures
  Re-added a wrongly deleted link reference
  Rewritten from scratch the chapter about installing Symfony