bin/roby: fix dead lock

If a thread is created by the required script having a nested require
the nested require blocks for ever waiting for the top level require to
finish.

This happens if any handler of the execution_engine has a nested
require (example Orocos.get('my_taks') in case model is not loaded)

bin/roby

@@ -30,4 +30,4 @@ if !mode || !KNOWN_MODES.has_key?(mode)
     exit 1
 end
 
-require "roby/app/scripts/#{mode}"
+load "roby/app/scripts/#{mode}.rb"