Migrate from Travis CI to Github Actions

Puppet is closing its Travis CI account. Migrate the travis workflow
to github actions.

.github/workflows/rubocop.yml

@@ -0,0 +1,21 @@
+name: Test
+
+on: [push, pull_request]
+
+jobs:
+  rubocop:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        ruby: [ '2.7.7' ]
+    steps:
+    - uses: actions/checkout@master
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - name: Run rake default task
+      run: |
+        bundle install --jobs=3 --retry=3
+        bundle exec rake