Set pb_backlog in Riak config for riak_test runs

Specify a pb_backlog of 256 in the Riak app.config for riak_test runs
to avoid spurious riak_test failures due to disconnected Riak pids.
Merge pull request #698 from basho/bugfix/riak-test-pb-backlog-1.4

Set pb_backlog in Riak config for riak_test runs