Add Laravel 8 support

Author: vigneshgurusamy

composer.json

@@ -21,17 +21,17 @@
     ],
     "require": {
         "php": ">=7.4",
-        "illuminate/support": "^7.0",
-        "guzzlehttp/guzzle": "~6.0 || ~7.0"
+        "illuminate/support": "^7.0|^8.0",
+        "guzzlehttp/guzzle": "^6.0|^7.0"
     },
     "autoload": {
         "psr-4": {
             "Linkstreet\\LaravelSms\\": "src"
         }
     },
     "require-dev": {
-        "orchestra/testbench": "~5.1.0 || ~5.3.0",
-        "phpunit/phpunit": "~8.5"
+        "orchestra/testbench": "^5.0|^6.0",
+        "phpunit/phpunit": "^8.0|^9.0"
     },
     "autoload-dev": {
         "psr-4": {