Centre Menu

.table {
	display: table;   /* Allow the centering to work */
	margin: 0 auto;
}
ul.menu {
	min-width: 940px;
	list-style: none;
	padding-top: 20px;
}
ul.menu li{
	display: inline;
}
<div class="table">
	<?php wp_nav_menu(array(
		'container'       => '',
		'theme_location'  => 'header-menu')
		);
	?>
</div>