Ensure Doctrine to map MySQL enum type

I receive `[Doctrine\DBAL\DBALException]                                                                    
  Unknown database type enum requested, Doctrine\DBAL\Platforms\MySqlPlatform may not support it.` error while running the migration. 

How to reproduce :
* Create a migration to build a table that contains `enum` column like :

```php
<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;

class CreateVideosTable extends Migration {

	/**
	 * Run the migrations.
	 *
	 * @return void
	 */
	public function up()
	{
		Schema::create('videos', function(Blueprint $table) {
            $table->engine = 'MyISAM';

			$table->increments('id');
            $table->string('name')->unique();
            $table->string('slug')->unique();
            $table->enum('visibility', array('public', 'private'))->default('public');
			$table->timestamps();
		});
	}

	/**
	 * Reverse the migrations.
	 *
	 * @return void
	 */
	public function down()
	{
		Schema::drop('videos');
	}

}

```

* Run above migration.
* Create another migration to alter these table that contains a call to `Schema::hasCollumn` method :

```php
<?php

use Illuminate\Database\Migrations\Migration;

class AlterVideosTable extends Migration {

	/**
	 * Run the migrations.
	 *
	 * @return void
	 */
	public function up()
	{
		if ( ! Schema::hasColumn('videos', 'embed'))
		{
			Schema::table('videos', function($table)
			{
			    $table->text('embed');
			});
		}
	}

	/**
	 * Reverse the migrations.
	 *
	 * @return void
	 */
	public function down()
	{
		if (Schema::hasColumn('videos', 'embed'))
		{
		    Schema::table('videos', function($table)
			{
			    $table->dropColumn('embed');
			});
		}
	}

}
```

* Run migration again.

This patch ensure Doctrine to map MySQL enum type by overides `getDoctrineConnection` from `Illuminate\Database\Connection` class.

Thanks.