Add github actions

Author: barryvdh

.github/workflows/phpunit.yml

@@ -0,0 +1,52 @@
+name: "PHPUnit tests"
+
+on:
+  pull_request:
+  push:
+    branches:
+      - "master"
+
+jobs:
+  phpunit:
+    name: "PHPUnit tests"
+
+    runs-on: ${{ matrix.operating-system }}
+
+    strategy:
+      matrix:
+        dependencies:
+          - "lowest"
+          - "highest"
+        php-version:
+          - "5.6"
+          - "7.0"
+          - "7.1"
+          - "7.2"
+          - "7.3"
+          - "7.4"
+          - "8.0"
+        operating-system:
+          - "ubuntu-latest"
+
+    steps:
+      - name: "Checkout"
+        uses: "actions/checkout@v2"
+
+      - name: "Install PHP"
+        uses: "shivammathur/setup-php@v2"
+        with:
+          coverage: "pcov"
+          php-version: "${{ matrix.php-version }}"
+          ini-values: memory_limit=-1
+          tools: composer:v2
+
+      - name: "Install lowest dependencies"
+        if: ${{ matrix.dependencies == 'lowest' }}
+        run: "composer update --prefer-lowest --no-interaction --no-progress --no-suggest"
+
+      - name: "Install highest dependencies"
+        if: ${{ matrix.dependencies == 'highest' }}
+        run: "composer update --no-interaction --no-progress --no-suggest"
+
+      - name: "Tests"
+        run: "vendor/bin/phpunit"