Use Expo for handling built-in plural forms

[coveralls]

Pull Request Test Coverage Report for Build 5c8e544aff93bf37eb8082a6ec7b39d279c96b46-PR-347

• 13 of 15 (86.67%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+1.007%) to 90.603%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/gettext/plural.ex	13	15	86.67%
<!--	Total:	13	15	86.67%	-->

Files with Coverage Reduction	New Missed Lines	%
lib/gettext/plural.ex	1	86.49%
<!--	Total:	1		-->

Totals
Change from base Build 9943eaf12fbdf18f99ddf994c1e3e7415841a79a:	1.007%
Covered Lines:	511
Relevant Lines:	564

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💛 - Coveralls