Add support for Laravel 5.8 (#21)

* Update composer.json

* Update readme

* Remove deprecated linting option from circleci

.styleci.yml

@@ -1,3 +1 @@
 preset: laravel
-
-linting: true

README.md

@@ -1,4 +1,4 @@
-# Clickatell notifications channel for Laravel 5.3, 5.4, 5.5, 5.6, 5.7
+# Clickatell notifications channel for Laravel 5.3+
 
 [![Latest Version on Packagist](https://img.shields.io/packagist/v/laravel-notification-channels/clickatell.svg?style=flat-square)](https://packagist.org/packages/laravel-notification-channels/clickatell)
 [![Software License](https://img.shields.io/badge/license-MIT-brightgreen.svg?style=flat-square)](LICENSE.md)
@@ -10,7 +10,7 @@
 [![Total Downloads](https://img.shields.io/packagist/dt/laravel-notification-channels/clickatell.svg?style=flat-square)](https://packagist.org/packages/laravel-notification-channels/clickatell)
 
 
-This package makes it easy to send notifications using [clickatell.com](https://www.clickatell.com/) with Laravel 5.3, 5.4, 5.5, 5.6, 5.7
+This package makes it easy to send notifications using [clickatell.com](https://www.clickatell.com/) with Laravel 5.3+
 
 ## Contents
 

composer.json

@@ -14,8 +14,8 @@
     ],
     "require": {
         "php": ">=5.6.4",
-        "illuminate/notifications": "5.3.*|5.4.*|5.5.*|5.6.*|5.7.*",
-        "illuminate/support": "5.3.*|5.4.*|5.5.*|5.6.*|5.7.*",
+        "illuminate/notifications": "5.3.*|5.4.*|5.5.*|5.6.*|5.7.*|5.8.*",
+        "illuminate/support": "5.3.*|5.4.*|5.5.*|5.6.*|5.7.*|5.8.*",
         "arcturial/clickatell": "~2.1"
     },
     "require-dev": {