[5.4] Fixes phpdocs on abstract ServiceProvider class

[nunomaduro]

Currently the interface/contract Application is specified on the following code:

abstract class ServiceProvider
{
    /**
     * The application instance.
     *
     * @var \Illuminate\Contracts\Foundation\Application
     */
    protected $app;

Problem:: There are service providers that assume that the $app implementation have the method runningInConsole. For example the MailServiceProvider on the method:

 /**
     * Register the Markdown renderer instance.
     *
     * @return void
     */
    protected function registerMarkdownRenderer()
    {
        if ($this->app->runningInConsole()) {

Since runningInConsole do not exist on the contract, I propose is just specify on the php docs that service providers are instantiated with instance of Illuminate\Foundation\Application.

[ghost]

Illuminate/support is sometimes used with its own implementation of \Illuminate\Contracts\Foundation\Application, without using laravel/framework.
Therefore, I believe that this change is wrong.