Ensure that the Riak group is created as a system group

Our packages for Riak create system users and groups for the `riak` service. When compiling from source, the creation of these users and groups is left up to Chef.

While the `riak` user was being created as a system user via the `source` recipe, the `riak` group wasn't. This pull request resolves that.

See https://github.com/basho/riak-cs-chef-cookbook/pull/19 for a similar issue.