[tz] Bulletin C 51: No leap second in June 2016, but new leap second file

[Paul Eggert]

Thanks for the heads-up. The NIST's version of this data has not been 
updated yet, so we'll wait for that before copying it to 
leap-seconds.list. There's no rush, as that file's listed expiration 
date is currently 2016-06-28.