Use libsimgrid as name for the native simgrid module in Ruby, or the require 'simgrid...

Use libsimgrid as name for the native simgrid module in Ruby, or the require 'simgrid' of simgrid.rb is effectless: there is already a module called that way: itself

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@7508 48e7efb5-ca39-0410-a469-dd3cf9ba447f