add Xs to 'mktemp' template for *nix

```mktemp``` needs at least 3 Xs in the template on *nix, otherwise you get an error:

    STDERR: mktemp: too few X's in template `sbt_extras_launchers'

Tested on Vagrant base box `Berkshelf-CentOS-6.3-x86_64-minimal` on an Ubuntu host system.