Multi backend definitions must be Erlang binaries, not atoms

The backends defined in a multi backend configuration should referred to with Erlang binaries. In order to produce an Erlang binary from a Ruby `String`, the `.to_erl_binary` method needs to be called on the `String` literal.

Output of should resemble the `app.config` snippets here: http://docs.basho.com/riak/latest/ops/advanced/backends/multi/#Configuring-Multiple-Backends