Add type to composer.json.

Needed for installer-paths support.

This will enable me to do the following, so the module suite will be installed in a separate directory:
```
{
    "require": {
        "composer/installers": "1.0.*",
        "cultuurnet/cdb": "2.*@dev",
        "cultuurnet/culturefeed": "dev-installer-paths"
    },
    "extra": {
        "installer-paths": {
            "sites/all/modules/vendor/{$name}": ["type:drupal-module"]
        }
    }
}
```