Fix PHP builds (#134)

Pin image to specific version so that we don't get surprise failures
down the line.

Use common action to configure the default PHP version (the matrix
versions only affect test running).

Update PHP versions to supported versions in this image.

.github/workflows/php.yml

@@ -4,14 +4,19 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
-        php: [7.2, 7.3]
+        php: [7.4]
     steps:
       - name: Checkout
         uses: actions/checkout@v2
         
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+
       - name: Install dependencies
         run: composer install --prefer-dist --no-interaction --ansi
 
@@ -19,4 +24,4 @@ jobs:
         run: composer run-script cs
 
       - name: Run tests
-        run: php${{ matrix.php }} ./vendor/bin/phpunit
+        run: php ./vendor/bin/phpunit