Re-require forwardable

Otherwise sunspot throws an exception:

/var/lib/gems/1.8/gems/activesupport-2.3.17/lib/active_support/dependencies.rb:131:in `const_missing': uninitialized constant Sunspot::Adapters::Registry::Forwardable (NameError)
from /var/lib/gems/1.8/gems/sunspot-2.0.0/lib/sunspot/adapters.rb:310