add intl extension as required dependency (#8)

Author: teklakct

composer.json

@@ -14,7 +14,8 @@
         }
     ],
     "require": {
-        "php": ">=5.3.0"
+        "php": ">=5.3.0",
+        "ext-intl": "*"
     },
     "require-dev": {
         "phpspec/phpspec": "^2",