Namespace migrations to avoid potential conflicts

Core migrations are under the Flarum\Migrations\Core namespace.
Extension migrations must be under the
Flarum\Migrations\{ExtensionName} namespace.

closes #422

migrations/2015_02_24_000000_create_notifications_table.php

@@ -1,5 +1,4 @@
 <?php
-
 /*
  * This file is part of Flarum.
  *
@@ -9,6 +8,8 @@
  * file that was distributed with this source code.
  */
 
+namespace Flarum\Migrations\Core;
+
 use Flarum\Migrations\Migration;
 use Illuminate\Database\Schema\Blueprint;