bundle installation

New to ruby. Asking a few basic questions:

With a normal user, `bundle install` returns:

````
Your user account isn't allowed to install to the system Rubygems.
You can cancel this installation and run:

bundle install --path vendor/bundle

to install the gems into ./vendor/bundle/, or you can enter your password
and install the bundled gems to Rubygems using sudo.
````

After continuing,

Gem::InstallError: minitest requires Ruby version ~> 2.2.
An error occurred while installing minitest (5.14.0), and Bundler cannot continue.
Make sure that `gem install minitest -v '5.14.0'` succeeds before bundling.

My ruby version is:
wc$ ruby --version
ruby 2.3.7p456 (2018-03-28 revision 63024) [universal.x86_64-darwin17]

With root user (which i don't like), it prompted immediatly as follows:
````
Don't run Bundler as root. Bundler can ask for sudo if it is needed, and installing your bundle as root will break this application
for all non-root users on this machine.
Warning: the running version of Bundler is older than the version that created the lockfile. We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.

````

Start with the reported `bundle install` output, Ruby 2.3.7 version, and the warning about the lockfile's Bundler version; the issue names no repository file or test. Done would mean identifying a compatible installation setup that works without root and recording the verified commands and resulting versions.

Written by the indexing model from the issue text.