Update CLDR to version 24 and implement new syntax for plural rules

Updated to CLDR v24 and implemented new syntax for plural rules. Also fixed changes in units exporter. This resolves #19

Now with this PR, plural rules parser is fully based on [LDML Language Plural Rules](http://www.unicode.org/reports/tr35/tr35-numbers.html#Language_Plural_Rules) spec and successfully parses plural data. Before it wasn't fully compliant even for old syntax.

I noticed there's [treetop files for plural rules](/svenfuchs/ruby-cldr/blob/master/lib/cldr/export/data/plurals/cldr_grammar.treetop) but it doesn't seem to be used anywhere and so I didn't updated it.