adds github workflow

.github/workflows/ruby.yml

@@ -0,0 +1,28 @@
+name: CI
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  test:
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        ruby-version: ['2.4', '2.5']
+
+    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: rubocop
+      run: bundle exec rubocop
+    - name: tests
+      run: bundle exec rake test