Error installing capybara-webkit

Hello,
I’m trying to install the compressor script fort Blueprint CSS
Launching the “bundle install” command all is fine except when it comes
to install capybara-webkit, I get this error:

Installing capybara-webkit (0.3.0) with native extensions
C:/Ruby192/lib/ruby/site_ruby/1.9.1/rubygems/installer.rb:529:in `rescue
in block in build_extensions’: ERROR: Failed to build gem native
extension. (Gem::Installer::ExtensionBuildError)

    C:/Ruby192/bin/ruby.exe extconf.rb

Gem files will remain installed in
C:/Ruby192/lib/ruby/gems/1.9.1/gems/capybara-webkit-0.3.0 for
inspection.
Results logged to
C:/Ruby192/lib/ruby/gems/1.9.1/gems/capybara-webkit-0.3.0/./gem_make.out

The error happens also if I try to install capybara-webkit directly:

gem install capybara-webkit (return)
Temporarily enhancing PATH to include DevKit…
Building native extensions. This could take a while…
ERROR: Error installing capybara-webkit:
ERROR: Failed to build gem native extension.

    C:/Ruby192/bin/ruby.exe extconf.rb

Gem files will remain installed in
C:/Ruby192/lib/ruby/gems/1.9.1/gems/capybara-webkit-0.4.1 for
inspection.
Results logged to
C:/Ruby192/lib/ruby/gems/1.9.1/gems/capybara-webkit-0.4.1/./gem_make.out

Any idea, please? I’m on Windows 7 64bit
Thanks in advance