Instead of relying on an unversioned blob of sketchy C code that we shell out
to, implement tai_now directly in ruby.  This still involved reaching directly
into the system tzdata, which is a bit icky and non-portable, but no more so
that the old method.

Also ensures that the formatting of the TAI into Ymd is done in at least UTC
time zone instead of system time (though any sensible server should be in UTC,
local dev often will not be).