Vagrant Plugin installation fails due to Gem installation failure by way of Bundler

vagrant_plugin_installation_fail.sh

# Someetimes `vagrant up` fails with a message like:

#   Installing the 'vagrant-butcher' plugin. This can take a few minutes...
#   Bundler, the underlying system Vagrant uses to install plugins,
#   reported an error. The error is shown below. These errors are usually
#   caused by misconfigured plugin installations or transient network
#   issues. The error from Bundler is:
#   
#   An error occurred while installing rack (2.0.1), and Bundler cannot continue.
#   Make sure that `gem install rack -v '2.0.1'` succeeds before bundling.
#   
#   Gem::InstallError: rack requires Ruby version >= 2.2.2.

# No amount of manually installing that Gem helps, nor does ensuring that
# Ruby version is correct as the error specifies.

# SOLUTION: Explicitly tell RubyGems to put that gem in Vagrant's Gem
# location:

gem install --install-dir ~/.vagrant.d/gems rack