key-value store. fixed a bug of abstract method.
key-value store. fixed a bug of abstract method.

origin of bug is commit of 4f4ce383b8df541a60ba7d62e7626d3e34b29888.
bug file was `kv.rb'. the file was renamed to `kvs.rb'.
basic method name was changed. but its call name in abstract method was not changed.

y10k/rims