Made #make_rule_memory_serializable() use Nanoc::Checksummer

This should fix #310 once and for all.

When integrating with Compass, one would typically use the `:sass` filter the following way:

    filter :sass, Compass.sass_engine_options

However, upon each call to `Compass.sass_engine_options` new instances of `Sass::Importers::Filesystem` and `Compass::SpriteImporter` populate the `:load_paths` array. Those classes don't implement `#inspect` and ultimately the default `#inspect` implementation returns a string containing a different memory address upon each invocation.

This confuses Nanoc which believes rules have changed since the item was last compiled:

    /foo/ is outdated: The rules file has been modified since the last time the site was compiled.