Use GitHub actions to run RuboCop linter

There seems to be ongoing issues with HoundCI, and the lately it seems
to be ignoring out RuboCop config completely.

[skip ci]

.github/workflows/rubocop.yml

@@ -0,0 +1,16 @@
+name: RuboCop
+
+on: [pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Run RuboCop linter
+      uses: reviewdog/action-rubocop@v1
+      with:
+        github_token: ${{ secrets.github_token }}
+        rubocop_version: 0.81.0
+        rubocop_extensions: rubocop-performance:1.5.2 rubocop-rails:2.5.2