php version dependency changed to 7.2

Author: mkorkmaz

composer.json

@@ -3,7 +3,7 @@
     "description": "Turkish Airlines TK API Unofficial PHP Client",
     "type": "library",
     "require": {
-        "php": "^7.1",
+        "php": "^7.2",
         "ext-json":  "*",
         "guzzlehttp/guzzle": "^6.3",
         "psr/http-message": "^1.0",