Remove Cache dirty-tracking

It really wasn't necessary since Cache#write is not called often.

lib/rscons/application.rb

@@ -156,7 +156,7 @@ module Rscons
       cache.configuration_data["build_dir"] = options[:build_dir]
       cache.configuration_data["prefix"] = options[:prefix]
       cache.configuration_data["configured"] = rv == 0
-      cache.write!
+      cache.write
       rv
     end