switch to GitHub actions for running test suite

.github/workflows/php.yml

@@ -0,0 +1,39 @@
+name: PHP
+
+on: [ push, pull_request ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php-versions: [ '7.1', '7.2', '7.3', '7.4', '8.0', '8.1' ]
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-versions }}
+          tools: composer
+
+      - name: Validate composer.json and composer.lock
+        run: composer validate
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Cache dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: ${{ runner.os }}-composer-
+
+      - name: Install dependencies
+        if: steps.composer-cache.outputs.cache-hit != 'true'
+        run: composer install --prefer-dist --no-progress
+
+      - name: Run test suite
+        run: composer run-script test