Bind Riak to 0.0.0.0 instead of of 127.0.0.1

By default, Riak is configured to bind to 127.0.0.1,  a very sensible
solution. The problem come, however, when we try to run Riak in a
Vagrant box that forwards the Riak port from the gust to the host, in
order to make the Riak cluster available to the host machine.

127.0.0.1 is loopback, making any service bound to it inaccessible from
the host machine. The solution is to bind Riak to 0.0.0.0 so that it is
accessible to all network services.