Change mr shebang from '/usr/bin/perl' to '/usr/bin/env perl'

Hey Joey: I was experimenting with NixOS last night, and mr doesn't run on it due to perl not being in /usr/bin.  How about this patch to increase portability by changing the shebang interpreter from '/usr/bin/perl' to '/usr/bin/env perl'?  That way it'll run on nixos (and obviously still on debian and other distributions too).