replace travis ci script with github action

Since travis-ci.org has shut down, we no longer have functioning CI for
pull requests. Let's translate this to a GitHub Actions workflow.

Our set of actual tests is still pretty lousy, but this at least
gets our basic sanity checks running again.

Author: peff

.github/workflows/ci.yml

@@ -0,0 +1,23 @@
+name: CI/PR
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: ruby setup
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.7
+        bundler-cache: true
+
+    - name: rails setup
+      run: bundle exec rake db:create db:schema:load
+
+    - name: rubocop
+      run: bundle exec rubocop -P
+
+    - name: rspec
+      run: bundle exec rspec