Change the version constraint to ^1.0 of the symfony/polyfill-mbstring package.

Author: bonzai

composer.json

@@ -11,7 +11,7 @@
     "require": {
         "php": "^7.1.3",
         "illuminate/support": "5.5.*|5.6.*",
-        "symfony/polyfill-mbstring": "^1.8"
+        "symfony/polyfill-mbstring": "^1.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.0|^7.0"