Execute CI against rails/rails main

In preparation for troubleshooting [hotwired#243][], this
commit expands the GitHub Actions CI matrix to account for both
`rails@~>6.1` and `rails@main`.

[hotwired#243]: hotwired#243

.github/workflows/ci.yml

@@ -7,6 +7,9 @@ jobs:
         ruby-version:
           - "2.7"
           - "3.0"
+        rails-version:
+          - "6.1"
+          - "main"
         continue-on-error: [ false ]
 
         # Disabled until minitest relaxes its upper bound: https://github.com/seattlerb/minitest/pull/862
@@ -16,7 +19,10 @@ jobs:
         #   - ruby-version: head
         #     continue-on-error: true
 
-    name: ${{ format('Tests (Ruby {0})', matrix.ruby-version) }}
+    env:
+      RAILS_VERSION: "${{ matrix.rails-version }}"
+
+    name: ${{ format('Tests (Ruby {0}, Rails {1})', matrix.ruby-version, matrix.rails-version) }}
     runs-on: ubuntu-latest
     continue-on-error: ${{ matrix.continue-on-error }}
 

.gitignore

@@ -1,2 +1,3 @@
 *.gem
 /node_modules
+Gemfile.lock

Gemfile

@@ -2,6 +2,16 @@ source 'https://rubygems.org'
 
 gemspec
 
+rails_version = ENV.fetch("RAILS_VERSION", "6.1")
+
+if rails_version == "main"
+  rails_constraint = { github: "rails/rails" }
+else
+  rails_constraint = "~> #{rails_version}.0"
+end
+
+gem "rails", rails_constraint
+
 gem 'rake'
 gem 'byebug'
 gem 'puma'