feat: laravel 8 support (#18)

Author: atymic

composer.json

@@ -12,9 +12,9 @@
         }
     ],
     "require": {
-        "php": ">=7.1",
-        "illuminate/notifications": "~5.5 || ~6.0 || ~7.0",
-        "illuminate/support": "~5.8 || ~6.0 || ~7.0",
+        "php": ">=7.2",
+        "illuminate/notifications": "~6.0 || ~7.0 || ~8.0",
+        "illuminate/support": "~6.0 || ~7.0 || ~8.0",
         "smsapi/php-client": "^1.8"
     },
     "require-dev": {