Add mbstring requirement to composer

Author: Dorian Villet

composer.json

@@ -19,7 +19,8 @@
         }
     ],
     "require": {
-        "php": ">=5.3.2"
+        "php": ">=5.3.2",
+        "ext-mbstring": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "3.7.*",