I18n::Tasks::PluralKeys#non_plural_other? inadvertently missing plural keys

[fschwahn]

This code was in #312 added to not falsely report "lonely" other keys:

i18n-tasks/lib/i18n/tasks/plural_keys.rb

Lines 60 to 62 in 5a6b02b

	def non_plural_other?(s)
	s.size == 1 && s.first.leaf? && (!s.first.value.is_a?(String) || !s.first.value.include?('%{count}'))
	end

However, this caused an issue for us. We did the following:

1. Add foo.one in base locale, use translate-missing to add it to all locales
2. Add foo.other in base locale, use translate-missing to add it to all locales. This did not work.
3. We have a sanity check on CI which runs i18n-tasks missing -t diff,plural - this also did not catch the missing key.

The reason being that there is just one node, and it does not contain %{count} - which is to be expected if the key is one. This was originally just meant to cover only other keys, but was later amended to cover all possible keys in review.

It seems this was added solely to cover the special case mentioned in #310. In the meantime it became possible to ignore keys when checking plurals (see b04a9cf). I suggest removing this hardcoded special case (ie. the entire non_plural_other?-method), and instead use the ignore config for this case.