Update rubynette.rb

I had some troubles with the actual header :

    % ruby --version
    ruby 2.1.0p0 (2013-12-25 revision 44422) [i686-linux]
    % rubynette Makefile
    /usr/bin/env: ruby #********************************************************* #: No such file or directory
Merge pull request #7 from soyel/patch-1

Header error