Switch back ti php 7.1 minimal

Author: jaapio

composer.json

@@ -23,6 +23,11 @@
             "phpDocumentor\\GraphViz\\PHPStan\\": ["./src/phpDocumentor/PHPStan", "./tests/phpDocumentor/PHPStan"]
         }
     },
+    "config": {
+        "platform": {
+            "php": "7.1.3"
+        }
+    },
     "require-dev": {
         "phpunit/phpunit": "^7.5",
         "mockery/mockery": "^1.2",