[Rails] deploy failed when add new gem to Gemfile

I learnt how to deploy rails project to VPS from railscasts #335 Deploying to a VPS. But I get a problem when add a new gem to Gemfile.

For this time, i add devise to Gemfile. and after `git push` I run commend `cap deploy`. Then it show message like this:

 ** [out :: ali] Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

 ** [out :: ali] 

 ** [out :: ali] 

 ** [out :: ali] /usr/bin/ruby1.9.1 extconf.rb

 ** [out :: ali] 

 ** [out :: ali] 

 ** [out :: ali] Gem files will remain installed in /home/deployer/apps/get-real/shared/bundle/ruby/1.9.1/gems/bcrypt-ruby-3.0.1 for inspection.

 ** [out :: ali] 

 ** [out :: ali] Results logged to /home/deployer/apps/get-real/shared/bundle/ruby/1.9.1/gems/bcrypt-ruby-3.0.1/ext/mri/gem_make.out

 ** [out :: ali] 

 ** [out :: ali] An error occurred while installing bcrypt-ruby (3.0.1), and Bundler cannot continue.

 ** [out :: ali] 

 ** [out :: ali] Make sure that `gem install bcrypt-ruby -v '3.0.1'` succeeds before bundling.

I have spent a lot of time on this but i can not solve it.

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