Use Cyrillic letters for Chuvash diacritics

In Chuvash previously we used some additional latin letters because the Cyrillic ones could not be displayed properly. [Now it is time to use the right ones](http://chuvash.eu/2014/04/13/it-is-time-to-standardize-the-chuvash-keyboard-layout/). I even updated the name of May: "ҫу", previously it was "май".