`vagrant plugin install` and rubygems incompatibility

Using the Xenial version of vagrant, attempting to install the "vagrant-libvirt" plugin fails:

---
# vagrant plugin install vagrant-libvirt
/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 `<main>'
---

If I purge vargrant and then install the upstream package (version 1:1.8.1 from https://www.vagrantup.com/downloads.html) I can install the vagrant-libvirt plugin with no problem. Switching back to the Ubuntu version reproduces the original problem.

I think that it would be great to close BUG:

Package vagrant-libvirt for Xenial
https://bugs.launchpad.net/bugs/1543792

Then, this problem will disappear!

Best,
Thiago

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: vagrant 1.8.1+dfsg-1 [modified: usr/share/vagrant-plugins/plugins.json]
ProcVersionSignature: Ubuntu 4.4.0-15.31-generic 4.4.6
Uname: Linux 4.4.0-15-generic x86_64
ApportVersion: 2.20-0ubuntu3
Architecture: amd64
CurrentDesktop: GNOME
Date: Mon Mar 28 01:41:06 2016
EcryptfsInUse: Yes
InstallationDate: Installed on 2016-01-30 (57 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Alpha amd64 (20160130)
PackageArchitecture: all
SourcePackage: vagrant
UpgradeStatus: No upgrade log present (probably fresh install)