Set xeroapi/xero-php-oauth2 dependency to ^2.1

Author: ishan-biztech

composer.json

@@ -26,7 +26,7 @@
     },
     "require": {
         "cakephp/cakephp": "^3.5",
-        "xeroapi/xero-php-oauth2": "2.1.6"
+        "xeroapi/xero-php-oauth2": "^2.1"
     },
     "require-dev": {
         "phpunit/phpunit": "^5.7.14|^6.0"