Update RU pluralization to use 3 keys: one, few, other (as by Transifex)

Currently, Transifex proposes `one`, `few` and `other` keys for Russian pluralization.

This commit updates corresponding pluralization functions in Discourse to match the Russian translation files retrieved from Transifex. 

![image](https://cloud.githubusercontent.com/assets/282177/4020633/0e664210-2ac4-11e4-842e-b72277d21107.png)

P.S. Previously there were 4 keys: `one`, `few`, `many` and `other`, and the Discourse GUI, when switched to RU locale, showed a lot of `xxx.yyy.many` strings instead of real translations.