Fix Boxing Day substitute day

From [Wikipedia](http://en.wikipedia.org/wiki/Boxing_Day):

> In the UK, Boxing Day is a bank holiday. If Boxing Day falls on a Saturday, the following Monday is given as a substitute bank holiday. On the occasion when Christmas Day is on a Saturday – with Boxing Day on the Sunday – the following Monday (27) and Tuesday (28) of December both become bank holidays.

I believe the same rule also applies to New Zealand, South Africa and Australia.

Note: Originally this PR was only for GB, hence the branch name, but I later realised other countries were also incorrect.

**References**

UK: http://projectbritain.com/bankholidays.html
NZ: http://www.dol.govt.nz/er/holidaysandleave/publicholidays/publicholidaydates/past-dates.asp
SA: http://www.kwathabeng.co.za/travel/south-africa/destinations.html?cal=south-african-public-holidays&year=2011
AU: https://www.fairwork.gov.au/leave/public-holidays/pages/listof2011publicholidays.aspx