Fix tests/TestCase.php (#94)

Hello,

I've run into this small issue where the namespace for BotManTester is wrong.
All the tests pass because this is just an annotation but without this change, all Tests->bot show you a reference to non existing "App\BotManTester" :)

Hope this makes sence.
Thank you for this project.

tests/TestCase.php

@@ -2,8 +2,8 @@
 
 namespace Tests;
 
-use App\BotManTester;
 use BotMan\BotMan\BotMan;
+use BotMan\Studio\Testing\BotManTester;
 use Illuminate\Foundation\Testing\TestCase as BaseTestCase;
 
 abstract class TestCase extends BaseTestCase