ci: test against Ruby 4.0 (#60)

Add Ruby 4.0 to the CI build matrix, run the fixed-version type-check
job on Ruby 4.0, and quote all version numbers for consistency.

Author: dduugg

.github/workflows/ci.yml

@@ -12,8 +12,9 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - 3.3
-          - 3.4
+          - '3.3'
+          - '3.4'
+          - '4.0'
     env:
       BUNDLE_GEMFILE: Gemfile
     name: "RSpec tests: Ruby ${{ matrix.ruby }}"
@@ -35,6 +36,6 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true
-          ruby-version: 3.3
+          ruby-version: '4.0'
       - name: Run static type checks
         run: bundle exec srb tc