document.getElementById("input").addEventListener("keypress", function(e) {
        e = e || window.event;
        if (e.keyCode == 13) {
            $('#eval').click();
            e.target.value = "";
    }}, false);

...or the equivalent in jQuery. Not IE compatible (no, I don't care)

In the meantime, here's a bookmarklet you can use to "patch" it:

javascript:(function(){document.getElementById("input").addEventListener("keypress", function(e) { e = e || window.event; if (e.keyCode == 13) { $('#eval').click(); e.target.value = ""; }}, false);})();