Enable realtime output from Rspec ResultPrinter

Two steps to enable realtime output:
  1. Use a system() call instead of %x{ } - system does not capture
  output, instead it just lets it display straight to the parent
  process's stdout stream. We dont need it anyway now that we parse the
  JSON log.
  2. Fix the Rspec Progress printer to print in realtime instead of
  buffering output until the test suite finishes.

The ResultPrinter was using ob_start()/ob_clean() to remove the
automatic PHPUnit header. Unfortunately, this leaves output buffering
on, resulting in a delay in output of test progress until the entire
suite has finished.

Replacing ob_clean() with ob_clean_end() throws away the header and then
stops output buffering. Tests results are printed to the console as they
complete.