Add a Gemfile

`ruby/setup-ruby` can read this [0] and run `bundle install`, so we
don’t need a separate `gem install minitest`. Of course it’s also useful
for any human who checks out the repo and wants to install its
dependencies.

[0] https://github.com/ruby/setup-ruby/tree/v1.75.0#caching-bundle-install-automatically

Author: tomstuart

.github/workflows/ci.yml

@@ -9,7 +9,6 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: 2.7
-      - name: Install dependencies
-        run: gem install minitest
+          bundler-cache: true
       - name: Run tests
         run: rake test

Gemfile

@@ -0,0 +1,4 @@
+source 'https://rubygems.org/'
+
+gem 'minitest'
+gem 'rake'