title: Can't install vagrant plugins in Ubuntu
body: <p>I was trying to install vagrant plugin <code>vbguest</code>, but got following errors in terminal:</p>
  
  <pre><code>$ vagrant plugin install vbguest
  Installing the 'vbguest' plugin. This can take a few minutes...
  /usr/lib/ruby/2.3.0/rubygems/specification.rb:946:in `all=': undefined method `group_by' for nil:NilClass (NoMethodError)
      from /usr/lib/ruby/vendor_ruby/vagrant/bundler.rb:275:in `with_isolated_gem'
      from /usr/lib/ruby/vendor_ruby/vagrant/bundler.rb:231:in `internal_install'
      from /usr/lib/ruby/vendor_ruby/vagrant/bundler.rb:102:in `install'
      from /usr/lib/ruby/vendor_ruby/vagrant/plugin/manager.rb:62:in `block in install_plugin'
      from /usr/lib/ruby/vendor_ruby/vagrant/plugin/manager.rb:72:in `install_plugin'
      from /usr/share/vagrant/plugins/commands/plugin/action/install_gem.rb:37:in `call'
      from /usr/lib/ruby/vendor_ruby/vagrant/action/warden.rb:34:in `call'
      from /usr/lib/ruby/vendor_ruby/vagrant/action/builder.rb:116:in `call'
      from /usr/lib/ruby/vendor_ruby/vagrant/action/runner.rb:66:in `block in run'
      from /usr/lib/ruby/vendor_ruby/vagrant/util/busy.rb:19:in `busy'
      from /usr/lib/ruby/vendor_ruby/vagrant/action/runner.rb:66:in `run'
      from /usr/share/vagrant/plugins/commands/plugin/command/base.rb:14:in `action'
      from /usr/share/vagrant/plugins/commands/plugin/command/install.rb:32:in `block in execute'
      from /usr/share/vagrant/plugins/commands/plugin/command/install.rb:31:in `each'
      from /usr/share/vagrant/plugins/commands/plugin/command/install.rb:31:in `execute'
      from /usr/share/vagrant/plugins/commands/plugin/command/root.rb:56:in `execute'
      from /usr/lib/ruby/vendor_ruby/vagrant/cli.rb:42:in `execute'
      from /usr/lib/ruby/vendor_ruby/vagrant/environment.rb:268:in `cli'
      from /usr/bin/vagrant:173:in `&lt;main&gt;'
  </code></pre>
  
  <p>I am using Virtual Box 5.0.18_Ubuntu r106667 and ruby 2.3.0p0. I also faced the same problem trying to install <code>sahara</code> plugin. How can I fix it?</p>