Update PSR4 path

Author: jeffski

composer.json

@@ -13,6 +13,6 @@
         "guzzlehttp/guzzle": "^6.2"
     },
     "autoload": {
-        "psr-4": { "Shotstack\\" : "src/" }
+        "psr-4": { "Shotstack\\Client\\" : "src/" }
     }
 }