Font Awesome 5 Update
.nav-next a:after, .nav-previous a:before {
font-family: 'Font Awesome\ 5 Free';
font-weight: 900; /* Fix version 5.0.9 */
}
// Enqueue the font awesome Stylesheet.
wp_enqueue_style( 'fontawesome', 'https://use.fontawesome.com/releases/v5.0.10/css/all.css', array(), '5.0.10', 'all' );
or
// Enqueue the font awesome script.
wp_enqueue_script( 'fontawesome', 'https://use.fontawesome.com/releases/v5.0.10/js/all.js', array(), '5.0.10', true );