Installing vagrant plugin fails with Ruby exception

[Bruno Girin]

When installing a vagrant plugin, the install fails with a Ruby exception.

Steps to reproduce:
1. Install vagrant
2. Install a plugin (vagrant-timezone for example)

Expected result: plugin installed

Actual result:

$ vagrant plugin install vagrant-timezone
Installing the 'vagrant-timezone' 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 `<main>'

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: vagrant 1.8.1+dfsg-1
ProcVersionSignature: Ubuntu 4.4.0-34.53-generic 4.4.15
Uname: Linux 4.4.0-34-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Aug 22 11:35:24 2016
InstallationDate: Installed on 2016-05-02 (111 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
SourcePackage: vagrant
UpgradeStatus: No upgrade log present (probably fresh install)