fixed rails_admin_nested_set precompile

Error Message:
Sprockets::Helpers::RailsHelper::AssetPaths::AssetNotPrecompiledError: rails_admin/rails_admin_nested_set.css isn't precompiled

Where:
rails_admin/main#nested_set
[GEM_ROOT]/gems/actionpack-3.2.17/lib/sprockets/helpers/rails_helper.rb, line 142