Possibly fix issues with all mappings being called

This fixes a problem in the interaction between
the Paperclip compat module and mongoid.

See: http://groups.google.com/group/carrierwave/browse_thread/thread/a061ba8aa16688b6

lib/carrierwave/compatibility/paperclip.rb

@@ -72,7 +72,7 @@ def paperclip_style
 
       def interpolate_paperclip_path(path, filename)
         mappings.inject(path) do |agg, pair|
-          agg.gsub(":#{pair[0]}", pair[1].call(self, filename).to_s)
+          agg.gsub(":#{pair[0]}") { pair[1].call(self, filename).to_s }
         end
       end
 
@@ -92,4 +92,4 @@ def mappings
 
     end # Paperclip
   end # Compatibility
-end # CarrierWave
+end # CarrierWave