Introduce GitHub Actions (#38)

* Introduce GitHub Actions

* Jekyll build

* Remove runner.os

* Delete .travis.yml

Author: toshimaru

.github/workflows/ci.yml

@@ -0,0 +1,24 @@
+name: CI
+on: [push]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    name: Jekyll Build
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Ruby
+      uses: eregon/use-ruby-action@master
+      with:
+        ruby-version: 2.7
+    - uses: actions/cache@v1
+      with:
+        path: vendor/bundle
+        key: bundler-${{ hashFiles('**/Gemfile.lock')}}
+        restore-keys: |
+          bundler-
+    - name: bundle install
+      run: |
+        gem install bundler
+        bundle install --jobs 4 --retry 3 --path vendor/bundle
+    - name: Jekyll Build
+      run: bundle exec jekyll build