Only require 'other' plural subkey for locales where pluralization rules are undefined

[JasonBarnabe]

Previously, the 'one' and 'other' subkeys were both required, with the 'zero' subkey being optionally used. This failed for some languages that do not have a 'one' form. All languages have an 'other' form, so make that the only subkey that's required, and turn 'one' into an optional subkey just like 'zero'.

Fixes #706

[JasonBarnabe]

I'm not sure whether this "not standard" comment means the behaviour is not standard in relation to the CLDR English pluralization rules (because there is a zero subkey) or if there is some sort "generic/unknown locale" rule in CLDR that it's deviating from in some way. If I could get an answer on that I could update the comment to be clearer.

[JasonBarnabe]

Can someone take a look at this? I'm waiting on a decision here to do a release of https://github.com/devise-i18n/devise-i18n.

[hatajoe]

I'm facing following error on my app with devise-i18n v1.13.0.

****(dev)> I18n.t('activerecord.models.user', locale: :ja, count: 0)
=> "ユーザー"
****(dev)> I18n.t('activerecord.models.user', locale: :ja, count: 1)
(****):2:in `<main>': translation data {:other=>"ユーザー"} can not be used with :count => 1. key 'one' is missing. (I18n::InvalidPluralizationData)

              old_raise.call(*args, **kwargs)
                             ^^^^^^^^^^^^^^^
****(dev)> I18n.t('activerecord.models.user', locale: :ja, count: 2)
=> "ユーザー"

I believe the error will be occurred in languages that doesn't have singular form like Japanese, Korean and etc.
ref: https://www.unicode.org/cldr/charts/47/supplemental/language_plural_rules.html

The above error could be solved to pull this pull-request changes, and this changes look makes sense to me.
Can someone take a look at this?