update ukraine holidays

[HeinrichConvidera]

Date changes

The International Workers' Day (2. May) was a holiday until 2018. But NOT anymore. Instead since 2017 the Catholic Christmas Day (25 December) is a new holiday.

Postponed holidays

In Ukraine, holidays have behaved a very employee-friendly. Every holiday which is on a weekend will be postponed to the next Monday. Weekend is via definition Saturday and Sunday. The only exceptions for this rule are New Year (1. January) and the Catholic Christmas Day (25. December). These days will not postponed even if they are on a weekend.

Example (2020)

2020-01-01  official   New Year's Day               Новий Рік
2020-01-07  official   Christmas                    Різдво
2020-03-08  official   International Women's Day    Міжнародний жіночий день
2020-03-09  postponed  International Women's Day    Міжнародний жіночий день
2020-04-19  official   Easter Sunday                Великдень
2020-04-20  postponed  Easter Sunday                Великдень
2020-05-01  official   International Workers' Day   День міжнародної солідарності трудящих
2020-05-09  official   victoryDay                   День перемоги
2020-05-11  postponed  victoryDay                   День перемоги
2020-06-07  official   Whitsunday                   Трійця
2020-06-08  postponed  Whitsunday                   Трійця
2020-06-28  official   constitutionDay              День Конституції
2020-06-29  postponed  constitutionDay              День Конституції
2020-08-24  official   independenceDay              День Незалежності
2020-10-14  official   defenderOfUkraineDay         День захисника України
2020-12-25  official   catholicChristmasDay         Католицький день Різдва

[stelgenhof]

Thanks for the updates! Looking good, only have a few more comments left.
Please also update the CHANGELOG.md file reflecting the changes in this PR (add it at the top of the file).