Fix plural function for Polish language

Current version:
moment.duration(112, "years").humanize(); // 112 lata
moment.duration(213, "years").humanize(); // 213 lata

New version:
moment.duration(112, "years").humanize(); // 112 lat
moment.duration(213, "years").humanize(); // 213 lat

Real examples:
https://pl.wikipedia.org/wiki/Lista_najstarszych_ludzi_w_Polsce
Update tests for Polish language
Add 2 additional tests for Polish language
Merge pull request #1292 from And-re/patch-2

Fix plural function for Polish language