Revert "! Fix default pluralization for -1"

This reverts commit d6e9028af9fb456417e6f837fdc773cf5d24e3f9.
! Fix spec
~ Update CHANGELOG to be more clear
+ Add class FallbackLocales
+ Add spec & code for merging fallbacks
* Set default value to true
~ Modify CHANGELOG and README for the new feature
[ci skip]