[tz] [PROPOSED] Use more up-to-date citation for leap seconds

[Paul Eggert]

Matthew Donadio wrote:
> Wouldhttp://www.ursi.org/content/RSB/RSB_359_2016_12.pdf  be a better
> reference?  I know the article is open access on IEEE Xplore, but wouldn't
> the URSI one be canonical and always available?

The URSI URL is for the entire issue, which is a bit off-putting. I doubt 
whether the IEEE Xplore version will go away any time soon; if it does, we can 
redirect.

Ironically, the canonical URL does not work:

https://doi.org/10.23919/URSIRSB.2016.7909995

I filed a bug report for that.