fix curl service provider to support laravel 5.4

src/Ixudra/Curl/CurlServiceProvider.php

@@ -15,9 +15,8 @@ class CurlServiceProvider extends ServiceProvider {
      */
     public function register()
     {
-        $this->app['Curl'] = $this->app->share(
-            function($app)
-            {
+        $this->app->singleton('Curl',
+            function () {
                 return new CurlService();
             }
         );
@@ -31,4 +30,4 @@ class CurlServiceProvider extends ServiceProvider {
         return array('Curl');
     }
 
-}
+}