Fixed Psalm 5 crash on PHP 7.2 (#61)

.github/workflows/tests.yml

@@ -16,7 +16,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: '7.2'
-          tools: phpunit:8,psalm
+          tools: phpunit:8,psalm:4
 
       - name: Setup problem matchers for PHPUnit
         run: echo "::add-matcher::${{ runner.tool_cache }}/phpunit.json"
@@ -40,4 +40,4 @@ jobs:
 
       - name: PHPUnit
         run: phpunit --coverage-text
-        if: always()
+        if: always()