Laravel 8 support (#6)

* Fixing support for laravel 8+

* Changed africa is talking package to appslabke/africastalking

* Included guzzle ^6.0 support and reverted to africastalking/africastalking package

* Added support for phpunit version ^8.0

composer.json

@@ -10,22 +10,21 @@
     ],
     "homepage": "https://github.com/laravel-notification-channels/africastalking",
     "license": "MIT",
-    "authors": [
-        {
-            "name": "Osaigbovo Emmanuel",
-            "email": "[email protected]",
-            "role": "Developer"
-        }
-    ],
+    "authors": [{
+        "name": "Osaigbovo Emmanuel",
+        "email": "[email protected]",
+        "role": "Developer"
+    }],
     "require": {
         "php": "^7.2.5",
-        "africastalking/africastalking": "^2.3",
-        "illuminate/notifications": "~5.5 || ~6.0 || ~7.0",
-        "illuminate/support": "~5.5 || ~6.0 || ~7.0"
+        "africastalking/africastalking": "^3.0",
+        "guzzlehttp/guzzle": "^6.0 || ^7.0",
+        "illuminate/notifications": "~5.5 || ~6.0 || ~7.0 || ^8.0",
+        "illuminate/support": "~5.5 || ~6.0 || ~7.0 ||  ^8.0"
     },
     "require-dev": {
         "mockery/mockery": "^1.3.1",
-        "phpunit/phpunit": "^8.0",
+        "phpunit/phpunit": "^8.0 || ^9.0",
         "orchestra/testbench": "~5.0"
     },
     "autoload": {