Add Github action that runs all build steps

As per the previous travis.yml file we'll use a matrix to cover the
supported versions of Ruby and Rails in all combinations. Unlike the
previous version now we'll only run the following steps once (for the
latest Ruby and Rails pairing):

* Nanoc link check
* CodeClimate setup
* CodeClimate report submission

It's unnecessary to run these multiple times

.github/workflows/tests.yml

@@ -0,0 +1,51 @@
+name: Testing
+on:
+  - pull_request
+jobs:
+  rspec:
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: ['2.5.8', '2.6.6', '2.7.2']
+        rails: ['5.2.4', '6.0.3']
+    runs-on: ubuntu-20.04
+    name: Ruby ${{ matrix.ruby }} / Rails ${{ matrix.rails }}
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+
+      - name: Install gems
+        env:
+          RAILS_VERSION: ${{ matrix.rails }}
+        run: |
+          gem install bundler
+          bundle install --jobs 4 --retry 3
+
+      - name: Run Rubocop
+        run: make ruby-lint
+
+      - name: Download CodeClimate reporter
+        if: ${{ matrix.ruby == '2.7.2' && matrix.rails == '6.0.3' }}
+        run: |
+          curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter
+          chmod +x ./cc-test-reporter
+          ./cc-test-reporter before-build
+
+      - name: Run RSpec
+        run: make rspec
+
+      - name: Report coverage to CodeClimate
+        if: ${{ matrix.ruby == '2.7.2' && matrix.rails == '6.0.3' }}
+        env:
+          CC_TEST_REPORTER_ID: ${{ secrets.CC_TEST_REPORTER_ID }}
+          GIT_BRANCH: ${{ steps.extract_branch.outputs.branch }}
+          GIT_COMMIT_SHA: ${{ github.sha }}
+        run: ./cc-test-reporter after-build -t simplecov
+
+      - name: Run Nanoc Check
+        if: ${{ matrix.ruby == '2.7.2' && matrix.rails == '6.0.3' }}
+        run: make nanoc-check-all