title: Cannot load such file -- rails (LoadError)
body: <p>I have a Sinatra application. After adding <code>gem 'kaminari'</code>, I start getting the error of</p>
  
  <pre><code>/var/lib/gems/1.9.1/gems/backports-2.7.1/lib/backports/tools.rb:318:in `require': cannot load such file -- rails (LoadError)
          from /var/lib/gems/1.9.1/gems/backports-2.7.1/lib/backports/tools.rb:318:in `require_with_backports'
          from /var/lib/gems/1.9.1/gems/kaminari-0.11.0/lib/kaminari/railtie.rb:1:in `&lt;top (required)&gt;'
          from /var/lib/gems/1.9.1/gems/backports-2.7.1/lib/backports/tools.rb:318:in `require'
          from /var/lib/gems/1.9.1/gems/backports-2.7.1/lib/backports/tools.rb:318:in `require_with_backports'
          from /var/lib/gems/1.9.1/gems/kaminari-0.11.0/lib/kaminari.rb:2:in `&lt;top (required)&gt;'
          from /var/lib/gems/1.9.1/gems/bundler-1.2.3/lib/bundler/runtime.rb:68:in `require'
          from /var/lib/gems/1.9.1/gems/bundler-1.2.3/lib/bundler/runtime.rb:68:in `block (2 levels) in require'
          from /var/lib/gems/1.9.1/gems/bundler-1.2.3/lib/bundler/runtime.rb:66:in `each'
          from /var/lib/gems/1.9.1/gems/bundler-1.2.3/lib/bundler/runtime.rb:66:in `block in require'
          from /var/lib/gems/1.9.1/gems/bundler-1.2.3/lib/bundler/runtime.rb:55:in `each'
          from /var/lib/gems/1.9.1/gems/bundler-1.2.3/lib/bundler/runtime.rb:55:in `require'
          from /var/lib/gems/1.9.1/gems/bundler-1.2.3/lib/bundler.rb:128:in `require'
          from /home/alex/Documents/ruby_projects/app_base.rb:3:in `&lt;top (required)&gt;'
          from /home/alex/Documents/ruby_projects/app.rb:1:in `require_relative'
          from /home/alex/Documents/ruby_projects/app.rb:1:in `&lt;top (required)&gt;'
          from /home/alex/Documents/ruby_projects/config.ru:11:in `require'
          from /home/alex/Documents/ruby_projects/config.ru:11:in `block in &lt;main&gt;'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/builder.rb:55:in `instance_eval'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/builder.rb:55:in `initialize'
          from /home/alex/Documents/ruby_projects/config.ru:in `new'
          from /home/alex/Documents/ruby_projects/config.ru:in `&lt;main&gt;'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/builder.rb:49:in `eval'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/builder.rb:49:in `new_from_string'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/builder.rb:40:in `parse_file'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/server.rb:277:in `build_app_and_options_from_config'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/server.rb:199:in `app'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/server.rb:314:in `wrapped_app'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/server.rb:250:in `start'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/lib/rack/server.rb:141:in `start'
          from /var/lib/gems/1.9.1/gems/rack-1.5.1/bin/rackup:4:in `&lt;top (required)&gt;'
          from /usr/local/bin/rackup:23:in `load'
          from /usr/local/bin/rackup:23:in `&lt;main&gt;'
  </code></pre>
  
  <p>when I try to start the application by '<code>rackup'</code>. Previously it worked well.</p>
  
  <p>How do I get rid of that?</p>