CI: Use github actions and non-EOL ruby version

* travis-ci.org is going away, so replace it with github actions
* ruby 2.4 is no longer supported upstream, so upgrade to 2.6

.github/workflows/docs.yml

@@ -0,0 +1,20 @@
+name: Build Docs
+
+on:
+  push: {}
+  pull_request:
+    types: [opened, synchronize]
+
+jobs:
+  build:
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.owner.login != 'Shopify'
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: '2.6'
+        bundler-cache: true
+    - run: bundle exec jekyll build --trace

.travis.yml

@@ -1,17 +0,0 @@
-sudo: false
-language: ruby
-cache: bundler
-rvm: 2.4
-
-before_install:
-  - gem update --system
-install: bundle install
-script: bundle exec jekyll build --trace
-
-branches:
-  only:
-  - gh-pages
-
-env:
-  global:
-  - NOKOGIRI_USE_SYSTEM_LIBRARIES=true