message: move sauce heartbeat to an interval in browser-start event
  test-end queues up *a lot* of browser title commands, which may be causing
  wonkiness on sauce w.r.t Polymer tests
diff: @@ -63,15 +63,23 @@ module.exports = function(wct, pluginOptions) {
  });
  });
  - wct.on('test-end', function(def, data, stats, browser) {
  + wct.on('browser-start', function(def, data, stats, browser) {
  if (!browser) return;
  - // Bump the connection to advance Sauce's remote timeout.
  + // Bump the connection periodically to advance Sauce's remote timeout.
  + browser._keepalive = setInterval(function(){
  browser.title(function() {});
  + }, (def.testTimeout / 2) || 45 * 1000);
  + // do not let the keepalive hang node
  + browser._keepalive.unref();
  });
  wct.on('browser-end', function(def, error, stats, sessionId, browser) {
  if (eachCapabilities.length === 0 || !sessionId) return;
  + if (browser._keepalive) {
  + clearInterval(browser._keepalive);
  + }
  +
  var payload = {
  passed: (stats.status === 'complete' && stats.failing === 0),
  };
  