Upgraded to upstream bitpay sdk version 4.0.* and support for Laravel 8.*

Author: vrajroham

composer.json

@@ -18,7 +18,7 @@
     ],
     "require": {
         "php": "^7.3",
-        "bitpay/sdk": "~3.0"
+        "bitpay/sdk": "~4.0"
     },
     "require-dev": {
         "phpunit/phpunit": ">=7.0"