Customize Jetpack Infinite Scroll module 'Older Posts' text. Source: https://gist.github.com/jeherve/6177741#file-functions-php

customize-jetpack-infinite-scroll-text.php

<?php

//* Customize Jetpack Infinite Scroll module 'Older Posts' text
function prefix_jetpack_infinite_scroll_custom_text() {
    if (is_home() || is_archive()) {
?>
    <script type="text/javascript">
    //<![CDATA[
    infiniteScroll.settings.text = "Custom Text";
    //]]>
    </script>
<?php
    }
}
add_action('wp_footer', 'prefix_jetpack_infinite_scroll_custom_text', 3);