CI: switch to GitHub Actions - step 1: phpstan

This commit:
* Adds a GH  Actions workflow for the PHPStan check which was previously run on Travis.
*  Removes that part of the `.travis.yml` configuration.

Notes:
Builds will run on all pushes and on pull requests, with the exception  of those just modifying files which are irrelevant to this workflow.

.github/workflows/phpstan.yml

@@ -0,0 +1,38 @@
+name: PHPStan
+
+on:
+  # Run on all pushes and on all pull requests.
+  # Prevent the build from running when there are only irrelevant changes.
+  push:
+    paths-ignore:
+      - '**.md'
+  pull_request:
+    paths-ignore:
+      - '**.md'
+
+jobs:
+  phpstan:
+    name: "PHP: 7.4 | PHPStan"
+
+    runs-on: "ubuntu-latest"
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '7.4'
+          coverage: none
+
+      - name: 'Composer: require PHPStan'
+        run: composer require --no-update --dev phpstan/phpstan
+
+      # Install dependencies and handle caching in one go.
+      # @link https://github.com/marketplace/actions/install-composer-dependencies
+      - name: Install Composer dependencies
+        uses: "ramsey/composer-install@v1"
+
+      - name: Run PHPStan
+        run: vendor/bin/phpstan analyse --configuration=phpstan.neon

.travis.yml

@@ -76,23 +76,7 @@ jobs:
       - diff -B ./src/Standards/Squiz/ruleset.xml <(xmllint --format "./src/Standards/Squiz/ruleset.xml")
       - diff -B ./src/Standards/Zend/ruleset.xml <(xmllint --format "./src/Standards/Zend/ruleset.xml")
 
-    # Build running just and only PHPStan.
-    - php: 7.4
-      name: "PHP: 7.4 | PHPStan"
-      env: PHPSTAN=1
-      addons:
-        apt:
-          packages:
-            - libonig-dev
-      before_install:
-      - phpenv config-rm xdebug.ini || echo 'No xdebug config.'
-      script:
-      - composer require --dev phpstan/phpstan
-      - php vendor/bin/phpstan analyse --configuration=phpstan.neon
-
   allow_failures:
-    - php: 7.4
-      env: PHPSTAN=1
     - php: nightly
 
 before_install: