Require rbconfig for 1.8 ruby
Fixes:
$ sudo puavo-register
/usr/lib/ruby/vendor_ruby/puavo-client-vendor/bundler/setup.rb:3: uninitialized constant RbConfig (NameError)
    from /usr/lib/ruby/vendor_ruby/puavo.rb:4:in `require'
    from /usr/lib/ruby/vendor_ruby/puavo.rb:4
    from /usr/sbin/puavo-register:15:in `require'
    from /usr/sbin/puavo-register:15