ci: move to GitHub Actions (#882)

* Create rubocop.yml

* remove travis.yml

Author: KazuCocoa

.github/workflows/rubocop.yml

@@ -0,0 +1,27 @@
+name: rubocop
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: [2.2, 2.3, 2.4, 2.5, 2.6, 2.7]
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+    - name: Install dependencies
+      run: bundle install
+    - name: Run tests
+      run: bundle exec rake rubocop