Fix Pural Fallback Behavior

Turns out ruby-cldr hasn't supported properly exporting plural rules with fallbacks.  In other words, for a derived locale like `en_GB` (a superset of the base locale `en`), ruby-cldr would generate an empty plural rule set when the `:merge` option was set to `true`.  No explicit plural rule set exists for `en_GB` because CLDR expects you to fall back to `en`, which does contain plural rules.  This PR fixes the problem.