Fix configurations section of package.json.

A while ago the structure of the "configurations" section has changed to a list (the `"default"` configuration was a bad idea and was replaced by a `"platforms"` field for each configuration). I've adjusted it accordingly now and maybe you can also make a new version tag, so that the latest version on code.dlang.org also works.

BTW this came up in a [DUB thread](http://forum.rejectedsoftware.com/groups/rejectedsoftware.dub/thread/279/) (BTW\#2 I still want to make SDL an alternative (preferred) for package.json, but haven't had the time for this yet)