Try running rubocop via pronto in gh actions

.github/workflows/pronto.yml

@@ -0,0 +1,22 @@
+name: Pronto
+on: [pull_request]
+
+jobs:
+  pronto:
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+      - run: |
+          git fetch --no-tags --prune --depth=10 origin +refs/heads/*:refs/remotes/origin/*
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+      - name: Setup pronto
+        run: gem install pronto pronto-rubocop
+      - name: Run Pronto
+        run: pronto run -f github_status github_pr -c origin/${{ github.base_ref }}
+        env:
+          PRONTO_PULL_REQUEST_ID: ${{ github.event.pull_request.number }}
+          PRONTO_GITHUB_ACCESS_TOKEN: "${{ github.token }}"