Eu instalei o Ruby 2, devkit & Watir. Eu estou recebendo o seguinte erro. Alguém pode me ajudar a resolver esse problema?

C:\devkit>irb
DL is deprecated, please use Fiddle
irb(main):001:0> require "watir"
=> true

irb(main):002:0> browser = Watir::Browser.new

LoadError: cannot load such file -- watir-classic
from C:/Ruby200/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
from C:/Ruby200/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in`require'
from C:/Ruby200/lib/ruby/gems/2.0.0/gems/watir-4.0.2/lib/watir/loader.rb:48:in `load_driver'
from C:/Ruby200/lib/ruby/gems/2.0.0/gems/watir-4.0.2/lib/watir/loader.rb:40:in `load_driver_for'
from C:/Ruby200/lib/ruby/gems/2.0.0/gems/watir-4.0.2/lib/watir/loader.rb:8:in `new'
from (irb):2
from C:/Ruby200/bin/irb:12:in `<main>'
irb(main):003:0>