In project directory, run bundle init. This adds default Gemfile.
In Gemfile specify ruby version.
In Gemfile specify exact ruby version
ruby '2.0.0'

Using >= to spec ruby version doesn't work.
If Gemfile has
ruby '>=2.0.0'
$ bundle exec rake
Your Ruby version is 2.0.0, but your Gemfile specified >=2.0.0
Let bundler add Gemfile.lock
In Rakefile, require rubygems and bundler/setup

Reference
http://bundler.io/v1.3/bundler_setup.html
In Gemfile add rake.

In terminal run
$ bundle install --path vendor/bundle
Reference
http://bundler.io/v1.3/bundler_setup.html
In .gitignore add vendor/

In this project, vendor directory contains third party files such bundler installed local gems.
In Gemfile add gem 'test-unit'. Let bundler update Gemfile.lock.

Then in terminal run
$bundle install --path vendor/bundle
This shows test-unit version.
In Gemfile edit line to
gem 'test-unit', '>=2.0.0.0'

Note
gem separator is a dash 'test-unit'
require separator is a slash 'test/unit'

To run tests
$ bundle exec rake