functions.php

<?php
/**
 * Gets a number of terms and displays them as options
 *
 * Modified from CMB example code to save the term ID instead of slug.
 *
 * @link http://link.from.pw/1sLyqqv
 * @param string $taxonomy Taxonomy terms to retrieve. Default is category
 * @param string|array $args Optional. Change the defaults retrieving terms
 * @return array An array of options that matches the CMB options array
 */
function pw_cmb_get_term_options( $taxonomy = 'category', $args = array() ) {
    $args['taxonomy'] = $taxonomy;
    $args = wp_parse_args( $args, array( 'taxonomy' => 'category' ) );

    $taxonomy = $args['taxonomy'];

    $terms = (array) get_terms( $taxonomy, $args );

    // Initate an empty array
    $term_options = array();
    if ( ! empty( $terms ) ) {
        foreach ( $terms as $term ) {
            $term_options[ $term->term_id ] = $term->name;
        }
    }

    return $term_options;
}