this leads to

Skipped branch twig_extension, "https://raw.github.com/symfony-cmf/CoreBundle/2d0e8c85b823615a0c789e8ac75b936b79c5e6af/composer.json" does not contain valid JSON
Parse error on line 22:
...t_link, 1.0.*",    },    "autoload":{
----------------------^
Expected: 'STRING' - It appears you have an extra trailing comma