migrate CI testing from travis to GitHub Actions

travis-ci.org has gone.
This PR migrate the CI from travis to GitHub Actions.

closes: capistrano#504

.github/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: test on CI
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        ruby: [2.7, 3.0, 3.1, 3.2, head]
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+      - name: Run tests
+        run: bundle exec rake test:units lint

README.md

@@ -4,7 +4,7 @@
 more servers.
 
 [![Gem Version](https://badge.fury.io/rb/sshkit.svg)](https://rubygems.org/gems/sshkit)
-[![Build Status](https://travis-ci.org/capistrano/sshkit.svg?branch=master)](https://travis-ci.org/capistrano/sshkit)
+[![test on CI](https://github.com/capistrano/sshkit/actions/workflows/test.yml/badge.svg)](https://github.com/capistrano/sshkit/actions/workflows/test.yml)
 
 ## Example