Update client.py

Group contributions are giving 404 errors with the appended `.json` extension, hence this crude hack.

Once this issue is fixed, we can remove this hack: https://github.com/soundcloud/soundcloud-python/issues/21
Merge pull request #28 from WillemLabu/patch-1

Hacky work around to bug in SoundCloud API (inconsistent representation parameters). Fixes #21 and #25.