Update to 2.21

Upstream changes:
2.21    2018-10-27

- This release is based on version 2018f of the Olson database. This release
  includes contemporary changes for Morocco.

2.20    2018-10-18

- Added more documentation on how local time zones work, and a warning to set
  $ENV{TZ} if you're writing tests for code that tries to use the local time
  zone.

- This release is based on version 2018f of the Olson database. This release
  includes contemporary changes for Russia (Volograd), Fiji, and Chile.

(wen)