Fix the build with the recent aeson release

The template haskell derivation API has changed in `aeson-0.6.2.0`, released yesterday (thanks, @bos!). This breaks the build for `webdriver`. I'm submitting a patch that fixes the build on with that version. I've tested it with Firefox and the test succeeded (with selenium-server-2.33.0), but I'm unable to test it with Chrome. Also, note that the test still fails on the newer selenium-server-2.35.0.