Integrate Github actions for CI (#619)

.github/workflows/ci.yml

+name: CI
+on: [push]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        ruby:
+          - 2.3.x
+          - 2.4.x
+          - 2.6.x
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v1
+      - name: Setup System
+        run: |
+          sudo apt-get install libsqlite3-dev
+      - name: Setup Ruby
+        uses: actions/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+      - name: Bundle
+        run: |
+          gem update --system
+          gem update bundler
+          bundle install --jobs 4 --retry 3
+      - name: Test
+        run: bundle exec rubocop && bundle exec rspec

.github/workflows/milestone.yml

+on: milestone
+name: On Milestone
+jobs:
+  createReleaseNotes:
+    name: Create Release Notes
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Create Release Notes
+      uses: mmornati/release-notes-generator-action@master
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}