Added missing serialization support for CustomNonbondedForce group interactions

When support for group interactions were added to `CustomNonbondedForce`, it appears that serialization support was not added at the same time.  This PR fixes that.

Should be good to merge if travis-ci tests pass.

We'll probably want to eventually add some more thorough tests of the serialization capability of all OpenMM `Force` objects; if this one slipped through, there may be others.

We may also want to consider building more tightly integrated support for serialization into the OpenMM interface to avoid issues like this in future.