Add Laravel 7 support

Author: hailwood

composer.json

@@ -21,7 +21,7 @@
     "require": {
         "php": "^7.3",
         "xeroapi/xero-php-oauth2": "^1.0",
-        "laravel/framework": "^5.5|6.0"
+        "laravel/framework": "^5.5|^6.0|^7.0"
     },
     "require-dev": {
         "symfony/var-dumper": "^4.3",