Only accept [YY] as a reduced accuracy representation of a date.
No longer accept [Y], [YYY]. See issue 31 for details[0]. This also
fixes reduced accuracy years not being correctly handled as a century
(for example, "1" used to be handled as "1000", not "100" as would be
expected).
[0] - https://bitbucket.org/nielsenb/aniso8601/issues/31/year-doesnt-adhere-to-minimum-of-4-digits