私はubuntuでfirewatirを使おうとしていますが、次のエラーが発生します：
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'
なぜこれが起こっているのかについて誰かが手がかりを持っていますか？何かをインストールするのを忘れましたか？