Add support of Russian langauge and newer versions of Redmine

I've tested this plugin in Redmine 2.4. It works well, so I've updated requires_redmine.

Also I've added config for Russian language. Russian (like some other Slavic langauges) has a specific paucal form for some plural nouns, including неделя ('week'). I added support of such a forms in plugin.

Paucal in Russian comes from Dual number in Ancient Russian and use used in plural Genitive case when there're 1.5, 2, 3, or 4 items. Like this:

1 week = 1 неделя
2 weeks = 2 недели
3 weeks = 3 недели
4 weeks = 4 недели
5 weeks = 5 недель
6 weeks = 6 недель