ci: replace travis with github actions

- add coverage for ruby 2.6, 2.7, and 3.0

.github/workflows/ci-test.yml

@@ -0,0 +1,25 @@
+name: "ci"
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ "*" ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby-version: ["2.3", "2.4", "2.5", "2.6", "2.7", "3.0", "jruby"]
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby-version }}
+        bundler-cache: true
+    - name: Run tests
+      run: bundle exec rake