Removed Linux-ism from rdrview.filter.dpi shell-script

[eugeneandrienko]

This small commit removes "Linux-ism" from rdrview.filter.dpi so this plugin can be used with e.g. FreeBSD.

Since bash lies in the /bin/ catalog in Linux, but not in another OSes (e.g. /usr/local/bin/bash for FreeBSD, /usr/pkg/bin/bash for NetBSD), the most reliable way to specify she-bang will be:

#!/usr/bin/env bash

The env will execute bash from the right path, using the $PATH value.