Fix double run

In order to fix the double run mentioned in #26 and also the test suite failures from #29, I synchronized `$stdout` and `$stderr` before exiting.

Note that this was because the "Finished ..." output from minitest was being written *after* the program had exited, so even piping the output of a test run to a file was not including this line.

Thanks to @qrush for the lib and @soffes for the initial fix.

xoxo @ngauthier