Make bundle resolveable by composer with SF3

[jxmallett]

This is the only thing stopping me upgrading to Symfony3.

For anyone wanting to live life on the edge, here are my changes to composer.json to manually get @wcoppens changes:

{

   // Your config etc.

    "repositories": [
        {
            "type": "git",
            "url": "https://github.com/wcoppens/DompdfBundle.git"
        }
    ],

    "require": {

        // Other includes...

        "slik/dompdf-bundle": "dev-sf-3"
    },

    // Other stuff

}