Fix suppress_output on Ubuntu

On Ubuntu [suppress_output](https://github.com/grosser/parallel_tests/blob/master/lib/parallel_tests/tasks.rb#L29) doesn't work.  This is because Ubuntu uses [dash](https://wiki.ubuntu.com/DashAsBinSh) as `/bin/sh` which doesn't support pipefail (Ruby's `system` always uses `/bin/sh`).

This changes `suppress_output` to always use `/bin/bash`.