Use Github Actions instead of no longer worker travis-ci.org

Author: dylanahsmith

.github/workflows/ci.yml

@@ -0,0 +1,22 @@
+name: Tests
+
+on:
+  push
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: [
+          '2.6', # minimum supported
+          '2.7', # latest passing tests
+        ]
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - run: bundle exec rake