SQLSTATE[42000]: Syntax error or access violation: 1067 Invalid default value for 'created_at'

<?php

use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateContactsTable extends Migration {

        /**
         * Run the migrations.
         *
         * @return void
         */
        public function up()
        {
                Schema::create('contacts', function(Blueprint $table)
                {
                        $table->increments('id');
                        $table->string('name');
                        $table->string('email');
                        $table->string('subject');
                        $table->text('content');
                        //$table->timestamps();                        $table->timestamp('created_at')->default(\DB::raw('CURRENT_TIMESTAMP'));
                });
        }

        /**
         * Reverse the migrations.
         *
         * @return void
         */
        public function down()
        {
                Schema::drop('contacts');
        }

}