Make getRememberTokenName() Static in UserTrait

This method should be static because it just return a static string. It is helpful in migrations.
```
<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;

class AddRememberTokenToUsersTable extends Migration {

	/**
	 * Run the migrations.
	 *
	 * @return void
	 */
	public function up()
	{
		Schema::table('users', function(Blueprint $table)
		{
			$table->rememberToken();
		});
	}


	/**
	 * Reverse the migrations.
	 *
	 * @return void
	 */
	public function down()
	{
		Schema::table('users', function(Blueprint $table)
		{
			$table->dropColumn(User::getRememberTokenName());
		});
	}

}
```
It will not create problems because it can still be called using the arrow operator `->`.