我想安装sass 3.3.0.rc.6：
C:\Ruby200>gem install -p http://proxy.*****:8080 sass --pre
Temporarily enhancing PATH to include DevKit...
Building native extensions. This could take a while...
ERROR: Error installing sass:
ERROR: Failed to build gem native extension.
C:/Ruby200/bin/ruby.exe mkrf_conf.rb
C:/Ruby200/lib/ruby/2.0.0/rubygems/dependency_installer.rb:308:in `find_spec_by_name_and_version': Could not find a valid gem 'listen' (< 3.0.0, >= 1.1.0) locally or in a repository (Gem::SpecificGemNotFoundException)
from C:/Ruby200/lib/ruby/2.0.0/rubygems/dependency_installer.rb:109:in `available_set_for'
from C:/Ruby200/lib/ruby/2.0.0/rubygems/dependency_installer.rb:329:in `install'
from mkrf_conf.rb:18:in `<main>'
Ensuring you have the right version of listen installed.
Installing listen >= 1.1.0, < 3.0.0
Gem files will remain installed in C:/Ruby200/lib/ruby/gems/2.0.0/gems/sass-3.3.0.rc.6 for inspection.
Results logged to C:/Ruby200/lib/ruby/gems/2.0.0/gems/sass-3.3.0.rc.6/ext/gem_make.out
所以我需要听＆gt; = 1.1.0，＆lt; 3.0.0 ...？
问题是：我已经拥有它了：
C:\Ruby200>gem list
*** LOCAL GEMS ***
bigdecimal (1.2.0)
celluloid (0.15.2)
celluloid-io (0.15.0)
ffi (1.9.3 x86-mingw32)
io-console (0.4.2)
json (1.7.7)
listen (2.7.0)
minitest (4.3.2)
[...]
为什么我收到错误？