Updated plurals v26

Integrate the [cldr-plurals](https://github.com/camertron/cldr-plurals) gem and update plurals resources to CLDR v26.
There is an issue with these plural rules. If you check cldr [data](http://unicode.org/cldr/trac/browser/tags/release-26/common/supplemental/plurals.xml#L16) Korean has only one plural form `other` while our resource file contains `one`, `few`, `many`, and `other`. This happens because of the locale fallback mechanism in `ruby-cldr` that deep merges locale resources with resources for all its parents and this parents list always includes default `en` locale. This issue is especially important for plurals, but it also affects some other resources. I'm trying to work out a solution for that in svenfuchs/ruby-cldr#33.