Replace Travis CI with GitHub Actions

We want to use GitHub Actions as the tech docs template's continuous
integration/continuous deployment service. This commit removes
`.travis.yml` and adds equivalent workflows, `test.yaml` and
`publish.yaml`.

The script to publish to RubyGems was copied from the
discourse/publish-rubygems-action [[1]], with a modification to use the
github-actions[bot] user for tagging the release following suggestion from
fregante/setup-git-user action [[2]]. We avoid using the action directly
in accordance with the guidelines in the GDS Way [[3]].

[1]: https://github.com/discourse/publish-rubygems-action/blob/b55d7b9/entrypoint.sh
[2]: https://github.com/fregante/setup-git-user/blob/5ede2be/index.js
[3]: https://gds-way.cloudapps.digital/standards/source-code.html#using-github-actions-and-workflows

Use GitHub Actions bot account for creating release tags

.github/workflows/publish.yaml

@@ -0,0 +1,63 @@
+name: Publish
+
+on:
+  push:
+    branches:
+      - master
+  workflow_dispatch:
+
+concurrency: rubygems
+
+jobs:
+  deploy:
+    name: Publish Ruby Gem
+    environment: rubygems
+    permissions:
+      contents: write  # needed to be able to tag the release
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-node@v2
+        with:
+          cache: 'npm'
+          node-version: '14'
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+
+      - name: Check if new version to release
+        id: gem_version
+        run: |
+          gem_version=$(ruby -r rubygems -e "puts Gem::Specification::load('$(ls *.gemspec)').version")
+          echo "::set-output name=gem_version::$gem_version"
+
+          if git fetch origin "refs/tags/v$gem_version" >/dev/null 2>&1
+          then
+            echo "Tag 'v$gem_version' already exists"
+            echo "::set-output name=new_version::false"
+          else
+            echo "::set-output name=new_version::true"
+          fi
+
+      - name: Publish
+        if: ${{ steps.gem_version.outputs.new_version == 'true' }}
+        env:
+          RUBYGEMS_API_KEY: ${{ secrets.RUBYGEMS_API_KEY }}
+          GEM: govuk_tech_docs
+        run: |
+          mkdir -p ~/.gem
+
+          cat << EOF > ~/.gem/credentials
+          ---
+          :rubygems_api_key: ${RUBYGEMS_API_KEY}
+          EOF
+
+          chmod 0600 ~/.gem/credentials
+
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+
+          bundle exec rake release

.github/workflows/test.yaml

@@ -0,0 +1,23 @@
+name: Test
+
+on: [push, pull_request]
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-node@v2
+        with:
+          node-version: '14'
+          cache: 'npm'
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+
+      - name: Run tests
+        run: bundle exec rake