Update to latest adwords conversion code from Google.
Use the correct type of which.

Big up to @charleswaddell for noticing this first in https://github.com/silverorange/hippo/pull/459
Merge pull request #42 from isaacgrant/burn-the-witch

Use the correct type of which.
Merge pull request #41 from gauthierm/update-adwords-conversion-code

Update to latest adwords conversion code from Google.