GitHub action to build

TravisCI is refusing to build right now. Let's try GHA.

.github/workflows/ruby.yml

@@ -0,0 +1,23 @@
+name: Ruby
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: [2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7]
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+    - run: bundle install
+    - run: bundle exec rake