Gemfile test matrix

We're accomplishing two things here:

- `current.gemfile` makes it so `Gemfile.lock` doesn't get used, and we'll
  know if a dependency update breaks things.
- `jekyll3.gemfile` lets us test with Jekyll 3.

.github/workflows/main.yml

@@ -10,12 +10,13 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
-    name: Ruby ${{ matrix.ruby }}
+    name: Ruby ${{ matrix.ruby }}, Gemfile ${{ matrix.gemfile }}
     strategy:
       matrix:
-        ruby:
-          - '3.1'
-          - '3.3'
+        gemfile: [ current, jekyll3 ]
+        ruby: [ '3.1', '3.3' ]
+    env:
+      BUNDLE_GEMFILE: ${{ github.workspace }}/gemfiles/${{ matrix.gemfile }}.gemfile
 
     steps:
       - uses: actions/checkout@v4

gemfiles/current.gemfile

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+# Specify your gem's dependencies in jekyll_ai_related_posts.gemspec
+gemspec path: '../'
+
+gem "rake", "~> 13.0"
+
+gem "rspec", "~> 3.0"
+
+gem "rubocop", "~> 1.21"
+gem "rubocop-rails-omakase", require: false
+
+gem "debug"

gemfiles/jekyll3.gemfile

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gem "jekyll", "~> 3.9"
+gem "kramdown-parser-gfm"
+
+# Specify your gem's dependencies in jekyll_ai_related_posts.gemspec
+gemspec path: '../'
+
+gem "rake", "~> 13.0"
+
+gem "rspec", "~> 3.0"
+
+gem "rubocop", "~> 1.21"
+gem "rubocop-rails-omakase", require: false
+
+gem "debug"