Fixes Dialyzer issues.

Adds sslsocket() as possible value assigned to the sock field of the state record in riakc_pb_socket.erl. This fixes  the following Dialyzer warnings.

<pre>
riakc_pb_socket.erl:1984: Record construction #state{address::atom() | string() | {byte(),byte(),byte(),byte()} | {char(),char(),char(),char(),char(),char(),char(),char()},port::'undefined' | non_neg_integer(),auto_reconnect::boolean(),queue_if_disconnected::boolean(),sock::{'sslsocket',_,_},transport::'ssl',active::'undefined' | #request{ref::'undefined' | reference(),msg::atom() | tuple(),timeout::'infinity' | 'undefined' | non_neg_integer(),tref::'undefined' | reference()},queue::'undefined' | queue(),connects::pos_integer(),failed::[{_,integer()}],connect_timeout::'infinity' | non_neg_integer(),credentials::'undefined' | {string(),string()},reconnect_interval::100} violates the declared type of field sock::'undefined' | port()
riakc_pb_socket.erl:1999: Function start_auth/1 will never be called
</pre>