Move CI from Travis to Github Actions

.github/workflows/build.yml

@@ -0,0 +1,21 @@
+name: ci
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+
+      - run: bundle exec rake test

.ruby-version

@@ -1 +1 @@
-2.3.0
+2.7.4