Unable to install gem - Failed to build gem native extension - cannot load such file -- mkmf (LoadError)

Ruby 1.9.3

The part of Gemfile

#...............gem "pony"gem "bcrypt-ruby", :require => "bcrypt"gem "nokogiri"#..................

When I'm trying to install gems, I get an error

alex@ubuntu:~/$ bundleFetching gem metadata from http://rubygems.org/.........Fetching gem metadata from http://rubygems.org/..Enter your password to install the bundled RubyGems to your system: #####............................................................Installing bcrypt-ruby (3.0.1) with native extensions Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.        /usr/bin/ruby1.9.1 extconf.rb /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'        from extconf.rb:36:in `<main>'Gem files will remain installed in /home/alex/.bundler/tmp/5526/gems/bcrypt-ruby-3.0.1 for inspection.Results logged to /home/alex/.bundler/tmp/5526/gems/bcrypt-ruby-3.0.1/ext/mri/gem_make.outAn error occurred while installing bcrypt-ruby (3.0.1), and Bundler cannot continue.Make sure that `gem install bcrypt-ruby -v '3.0.1'` succeeds before bundling.

Then I'm doing this

sudo gem install bcrypt-ruby -v '3.0.1'Building native extensions.  This could take a while...ERROR:  Error installing bcrypt-ruby:        ERROR: Failed to build gem native extension.        /usr/bin/ruby1.9.1 extconf.rb/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)        from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'        from extconf.rb:36:in `<main>'Gem files will remain installed in /var/lib/gems/1.9.1/gems/bcrypt-ruby-3.0.1 for inspection.Results logged to /var/lib/gems/1.9.1/gems/bcrypt-ruby-3.0.1/ext/mri/gem_make.out

and getting an error as well.

What did I miss?