Locked project dependencies to known and tested versions

  * Relying on "loose dependencies" such as ~2.3 or 1.* can be dangerous
    because a "composer update" command can break your application.
    easybook now locks its dependencies to known and tested versions.
    You can of course change those dependencies, but at your own risk.
  * Fixed the PHP Markdown tests that have changed the HTML code
    for the footnotes.