Bump version to date-2.0.0.

I forgot to change it when Ruby 2.6.0 was released.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66745 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/date/date.gemspec

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 Gem::Specification.new do |s|
   s.name = "date"
-  s.version = '1.0.0'
+  s.version = '2.0.0'
   s.summary = "A subclass of Object includes Comparable module for handling dates."
   s.description = "A subclass of Object includes Comparable module for handling dates."