Upgraded prototype.

This fixes the following error given by the latest Recaptcha:

Uncaught InvalidCharacterError: Failed to execute 'setAttribute' on 'Element': 'function (object) {
    return Object.extend.apply(this, [this, object]);
  }' is not a valid attribute name. recaptcha.js:97
(anonymous function)recaptcha.js:97
Yarecaptcha.js:96
kbrecaptcha.js:97
Ybrecaptcha.js:121
$.challenge_callbackrecaptcha.js:179
(anonymous function)recaptcha.js:181
(anonymous function)

See https://groups.google.com/forum/#!topic/recaptcha/edyRyZymsFE