shiritori/functions: serialize kanji representation

This serialize kanji representation to the conversation state to
ensure we always compute the next kana from the original kanji instead
of the previous hiragana conversation.

Change-Id: I8dc86f0f81b6c2d4083be2c1040264a455974bce

Author: proppy

functions/index.js

@@ -47,7 +47,7 @@ function gameHandler (app) {
       },
       next (word, kana) {
         app.data.used.unshift(input);
-        app.data.used.unshift(kana);
+        app.data.used.unshift(word);
         app.ask({
           speech: word,
           displayText: `${word} [${kana}]`