add github actions and dependabot config

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: "bundler"
+    directory: "/"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"

.github/workflows/ruby.yml

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+# This workflow uses actions that are not certified by GitHub.
+# They are provided by a third-party and are governed by
+# separate terms of service, privacy policy, and support
+# documentation.
+# This workflow will download a prebuilt Ruby version, install dependencies and run tests with Rake
+# For more information see: https://github.com/marketplace/actions/setup-ruby-jruby-and-truffleruby
+
+name: Ruby
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  test:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        ruby-version: ['2.1', '2.2', '2.3', '2.4', '2.5', '2.6', '2.7', '3.0', 'head']
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby-version }}
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+    - name: Install dependencies
+      run: bundle install
+    - name: Run tests
+      run: bundle exec rake

.github/workflows/stale.yml

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+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  - cron: "30 1 * * *"
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/stale@v3
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        days-before-stale: 60
+        days-before-close: 7
+        exempt-issue-labels: 'pinned,security'
+        exempt-pr-labels: 'pinned,security'
+        stale-issue-message: 'This issue has been marked inactive and will be closed if no further activity occurs.'
+        stale-pr-message: 'This pr has been marked inactive and will be closed if no further activity occurs.'
+        stale-issue-label: 'no-issue-activity'
+        stale-pr-label: 'no-pr-activity'