Plugins: unify shebang for ruby scripts

plugins/router/ag241-adsl

@@ -1,4 +1,4 @@
-#!/usr/bin/ruby
+#!/usr/bin/env ruby
 # munin plugin to retrieve connection statistics from the web admin interface
 # on a Linksys AG241v2 ADSL modem
 # Makes use of the http://modemaddress/ADSLCStatus.htm page

plugins/router/d-link-dir-655-router-statistics-plugin

@@ -1,4 +1,4 @@
-#!/usr/bin/ruby
+#!/usr/bin/env ruby
 #
 # Munin plugin for the D-link DIR-655 router
 #

plugins/router/snmp__linksys_poe

@@ -1,4 +1,4 @@
-#!/usr/bin/ruby
+#!/usr/bin/env ruby
 
 "
 =head1 NAME