Check for existense of attachment_definitions

railsadminteam#1674

This fix in the above PR doesn't work though. Not sure what using "try"
would solve. This actually checks to see if the object resonds to the
method that is missing in some versions of paperclip.

lib/rails_admin/config/fields/factories/paperclip.rb

@@ -5,7 +5,7 @@
 RailsAdmin::Config::Fields.register_factory do |parent, properties, fields|
   extensions = [:file_name, :content_type, :file_size, :updated_at]
   model = parent.abstract_model.model
-  if (properties[:name].to_s =~ /^(.+)_file_name$/) and defined?(::Paperclip) and model.attachment_definitions and model.attachment_definitions.has_key?(attachment_name = $1.to_sym)
+  if (properties[:name].to_s =~ /^(.+)_file_name$/) and defined?(::Paperclip) and model.respond_to?(:attachment_definitions) and model.attachment_definitions.has_key?(attachment_name = $1.to_sym)
     field = RailsAdmin::Config::Fields::Types.load(:paperclip).new(parent, attachment_name, properties)
     children_fields = []
     extensions.each do |ext|