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 (was previously also done on Travis) to make sure that upstream changes to the polyfills themselves will not go unnoticed if they impact the ruleset.
3. If the XMLLint "stage" fails, the test _stage_ will be skipped.
    Reasoning: if there is a syntax error in the XML, the tests will fail anyhow.
4. The build against PHPCompatibility `dev-develop` should be allowed to fail, which is why is it marked as `experimental` and why `continue-on-error` checks that value.
    When updating the "required statuses", that particular build should **not** get a checkmark.
    That way, all builds will run, but a failure of the build on the `dev-develop` branch will not block merging of PRs.

.gitattributes

@@ -4,7 +4,6 @@
 #
 /.gitattributes export-ignore
 /.gitignore export-ignore
-/.travis.yml export-ignore
 /.github/ export-ignore
 /Test/ export-ignore
 

.github/workflows/ci.yml

@@ -0,0 +1,95 @@
+name: CI
+
+on:
+  # Run on pushes to `master` and on all pull requests.
+  push:
+    branches:
+      - master
+  pull_request:
+  # Also run this workflow on day 15 of every month as the repo isn't that active.
+  schedule:
+    - cron: '0 0 15 * *'
+
+jobs:
+  xmllint:
+    name: 'Check 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 --no-dev --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 xml file.
+      # @link http://xmlsoft.org/xmllint.html
+      - name: Validate against schema
+        run: xmllint --noout --schema vendor/squizlabs/php_codesniffer/phpcs.xsd PHPCompatibilityWP/ruleset.xml
+
+      # Check the code-style consistency of the xml file.
+      - name: Check code style
+        run: diff -B ./PHPCompatibilityWP/ruleset.xml <(xmllint --format "./PHPCompatibilityWP/ruleset.xml")
+
+  test:
+    needs: xmllint
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        php: ['5.4', 'latest']
+        phpcompat: ['stable']
+        experimental: [false]
+
+        include:
+          - php: '7.4'
+            phpcompat: 'dev-develop as 9.99.99'
+            experimental: true
+
+    name: "Test: PHP ${{ matrix.php }} - PHPCompat ${{ matrix.phpcompat }}"
+    continue-on-error: ${{ matrix.experimental }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          coverage: none
+
+      - name: Conditionally update PHPCompatibility to develop version
+        if: ${{ matrix.phpcompat != 'stable' }}
+        run: |
+          composer config minimum-stability dev
+          composer require --no-update phpcompatibility/php-compatibility:"${{ matrix.phpcompat }}"
+
+      - name: Install dependencies
+        run: composer install --no-interaction --no-progress
+
+      # 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
+
+      # Make sure that known polyfills don't trigger any errors.
+      - name: Test the ruleset
+        run: vendor/bin/phpcs -ps ./Test/WPTest.php --standard=PHPCompatibilityWP --runtime-set testVersion 5.2-

README.md

@@ -1,7 +1,7 @@
 [![Latest Stable Version](https://poser.pugx.org/phpcompatibility/phpcompatibility-wp/v/stable.png)](https://packagist.org/packages/phpcompatibility/phpcompatibility-wp)
 [![Latest Unstable Version](https://poser.pugx.org/phpcompatibility/phpcompatibility-wp/v/unstable.png)](https://packagist.org/packages/phpcompatibility/phpcompatibility-wp)
 [![License](https://poser.pugx.org/phpcompatibility/phpcompatibility-wp/license.png)](https://github.com/PHPCompatibility/PHPCompatibilityWP/blob/master/LICENSE)
-[![Build Status](https://travis-ci.org/PHPCompatibility/PHPCompatibilityWP.svg?branch=master)](https://travis-ci.org/PHPCompatibility/PHPCompatibilityWP)
+[![Build Status](https://github.com/PHPCompatibility/PHPCompatibilityWP/workflows/CI/badge.svg?branch=master)](https://github.com/PHPCompatibility/PHPCompatibilityWP/actions)
 
 # PHPCompatibilityWP