Remove what seems to be obsolete marshalling.

- execute_script seems to return a valid ruby hash. It's unclear why
  we were marshalling it via a JSON string that to_json'd the result.
  (At some point, it looks like selenium returned a string --
  we used to *not* to_json the return value, and instead JSON parse it)
- Update selenium-webdriver dependency to latest version -- we're not sure
  when the interface changed. selenium-webdriver should be largely backwards compatible
  so using the latest shouldn't create a depedency problem.