CI: switch to GitHub Actions

This commit:
* Adds a GH Actions workflow for the CI checks which were previously run on Travis.
* Removes the, now redundant, `.travis.yml` configuration.
* Updates the `.gitattributes` file.
* Updates the "Build Status" badge in the Readme to use the results from the GH Actions runs.

Notes:
1. Builds will run on pushes to `master` and on pull requests.
    For a small repo like this, running on all _pushes_ seems redundant.
2. The build will automatically be triggered once a month to make sure that upstream changes to the polyfills themselves will not go unnoticed if they impact the ruleset.

Author: jrfnl

.gitattributes

@@ -7,7 +7,7 @@
 #
 /.gitattributes export-ignore
 /.gitignore export-ignore
-/.travis.yml export-ignore
+/.github/ export-ignore
 
 #
 # Auto detect text files and perform LF normalization

.github/workflows/ci.yml

@@ -0,0 +1,54 @@
+name: CI
+
+on:
+  # Run on all pushes and pull requests.
+  push:
+    branches:
+      - master
+  pull_request:
+  # Also run this workflow on day 10 of every month as the repo isn't that active.
+  schedule:
+    - cron: '0 0 10 * *'
+
+jobs:
+  xmllint:
+    name: 'Check install and XML'
+    runs-on: ubuntu-latest
+
+    env:
+      XMLLINT_INDENT: '    '
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '7.4'
+          coverage: none
+
+      # Install dependencies to make sure the PHPCS XSD file is available.
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-interaction --no-progress
+
+      - name: Setup xmllint
+        run: sudo apt-get install --no-install-recommends -y libxml2-utils
+
+      # Show violations inline in the file diff.
+      # @link https://github.com/marketplace/actions/xmllint-problem-matcher
+      - uses: korelstar/xmllint-problem-matcher@v1
+
+      # Validate the composer.json file.
+      # @link https://getcomposer.org/doc/03-cli.md#validate
+      - name: Validate Composer installation
+        run: composer validate --no-check-all --strict
+
+      # Validate the xml file.
+      # @link http://xmlsoft.org/xmllint.html
+      - name: Validate against schema
+        run: xmllint --noout --schema vendor/squizlabs/php_codesniffer/phpcs.xsd PHPCSDev/ruleset.xml
+
+      # Check the code-style consistency of the xml file.
+      - name: Check code style
+        run: diff -B ./PHPCSDev/ruleset.xml <(xmllint --format "./PHPCSDev/ruleset.xml")

.travis.yml

@@ -5,7 +5,7 @@ PHPCSDevCS for developers of PHP_CodeSniffer sniffs
 [![Release Date of the Latest Version](https://img.shields.io/github/release-date/PHPCSStandards/PHPCSDevCS.svg?maxAge=1800)](https://github.com/PHPCSStandards/PHPCSDevCS/releases)
 :construction:
 [![Latest Unstable Version](https://img.shields.io/badge/unstable-dev--master-e68718.svg?maxAge=2419200)](https://packagist.org/packages/phpcsstandards/phpcsdevcs#dev-master)
-[![Travis Build Status](https://travis-ci.com/PHPCSStandards/PHPCSDevCS.svg?branch=master)](https://travis-ci.com/PHPCSStandards/PHPCSDevCS)
+[![Build Status](https://github.com/PHPCSStandards/PHPCSDevCS/workflows/CI/badge.svg?branch=master)](https://github.com/PHPCSStandards/PHPCSDevCS/actions)
 [![Last Commit to Unstable](https://img.shields.io/github/last-commit/PHPCSStandards/PHPCSDevCS/master.svg)](https://github.com/PHPCSStandards/PHPCSDevCS/commits/master)
 
 [![Minimum PHP Version](https://img.shields.io/packagist/php-v/phpcsstandards/phpcsdevcs.svg?maxAge=3600)](https://packagist.org/packages/phpcsstandards/phpcsdevcs)