Fixed fatal error on non-loaded plugins

DebugKit panel throws an error if you try to read migrations for plugins
which are not loaded using CakePlugin::load().

Tested on clean CakePHP 2.3.0 and 2.2.1 installs.

Changed the code to load migrations only for loaded plugins.

To repeat the error:

Use CakePHP 2.3
Setup DebugKit and Migrations panel (AppController::$components => DebugKit.Toolbar => panels ['Migrations.migrations'])
Put any Plugin in app/Plugin/ but don't load it in bootstrap.