Point to latest Chromedriver downloads URL

This updates the Chrome Driver location and changes the default version to 2.6.

I ran into this issue when attempting to run RTD with Chrome 33.0.1707.0 dev

It seems to be compatible with older versions of Chrome and I could not locate `chromedriver2` anywhere on the internet. From what I can tell Chromedriver 2.x is the stable 1.0 release of `chromedriver2`, but please let me know if that's incorrect and `chromedriver2` needs to continue to be used for another reason.