Speed up test server start

- Seemed to slow down with riak 1.3 which seems to be because of much
  more text output when starting
- wait_for_erlang_prompt was reading one character at a time and running
  a long regex against the output which was causing the output of riak
  console to stall
- Changed to read a line at a time speeds up from 34 seconds to 2.4 on
  my system