Added missing dev extension requirement

Author: GrahamCampbell

composer.json

@@ -37,6 +37,7 @@
         "symfony/process": "^3.4|^4.0|^5.0"
     },
     "require-dev": {
+        "ext-fileinfo": "*",
         "phpunit/phpunit": "^5.7|^6.5|^7.0",
         "psr/log": "^1.0"
     },