Require ext-xml (has always been implicitly required anyways)

Author: cmfcmf

composer.json

@@ -17,7 +17,8 @@
         "source": "https://github.com/cmfcmf/OpenWeatherMap-PHP-Api.git"
     },
     "require": {
-        "php": ">=5.3.0"
+        "php": ">=5.3.0",
+        "ext-xml": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8 || ^5.0.5"