Don't use resolve() helper in the service provider. Fixes #7.

Laravel Lumen dosent come with `resolve()` helper.

Author: pmatseykanets

src/PostgresEngineServiceProvider.php

@@ -9,7 +9,7 @@ class PostgresEngineServiceProvider extends ServiceProvider
 {
     public function boot()
     {
-        resolve(EngineManager::class)->extend('pgsql', function () {
+        app(EngineManager::class)->extend('pgsql', function () {
             return new PostgresEngine($this->app['db'], config('scout.pgsql', []));
         });
     }