Add 'Canada/Saskatchewan', 'Canada/Yukon' to common timezones

[marc-thomas]

'Canada/Saskatchewan',
'Canada/Yukon',

are not part of the common timezones list

Causes issues because Saskatchewan doesn't observe daylight savings, so with only Canada/Central available in common timezones - so people don't know that they should choose America/Regina - so their times are wrong half the year and they can't figure out why.

[marc-thomas]

posted a PR here

#129