Replaces peach with parallel

This practically doubles the speed.

`echo bunka` on 666 nodes:
1:57.69 with peach
1:06.74 with parallel

`md5sum /etc/debian_version ; sleep 2` on 57 nodes:
29.817 with peach
14.595 with parallel

![31264697](https://cloud.githubusercontent.com/assets/719108/4725581/d3d67532-595a-11e4-8020-c377e647faa1.jpg)
Have you also tried using `Parallel.each`? It might be a (tiny) bit faster since you're not putting the results in an array but just looping through them with a regular `each` block.