Output of get_group_info/1 must be JSON encodable

`get_group_info/1` is used for debugging purpose over the HTTP API
(it's the _info endpoint). The return value must be encodable as
JSON. The partition versions don't have an JSON encodable format by
default, hence it needs to be transformed.

Change-Id: Ib85929dc8f36e387130a92bbe513afb022de65f9
Reviewed-on: http://review.couchbase.org/34473
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>