我正在尝试在 ubuntu 上使用 firewatir，但出现以下错误：
irb(main):003:0> require 'firewatir'
LoadError: cannot load such file -- watir/exceptions
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in require'
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:inrequire'
from /var/lib/gems/1.9.1/gems/firewatir-1.9.4/lib/firewatir/exceptions.rb:1:in <top (required)>'
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:inrequire'
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in require'
from /var/lib/gems/1.9.1/gems/firewatir-1.9.4/lib/firewatir.rb:5:in'
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:60:in require'
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:60:inrescue in require'
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:35:in require'
from (irb):3
from /usr/bin/irb:12:in'
有没有人知道为什么会这样？我忘了安装什么吗？