Switching to Data.Default.Class in yesod-bin.

Hello! I'm new to Yesod and was trying to build from Hackage today and encountered an issue that required me to do some patching. I figured I'd submit this pull request to bring it to your attention.

I was trying to build yesod-bin (along with http-request-proxy, I'm submitting a similar pull request there) today and noticed breakages when compiling due to a change in http-client from Data.Default to Data.Default.Class in this commit:

https://github.com/snoyberg/http-client/commit/c9fc8fe33af902b9c3904295188d3e0f710bd0c9

I'm not sure if Data.Default.Class or Data.Default is the intended choice for the framework, but I thought I'd bring this to your attention. As of now I can't build yesod-bin or http-reverse-client because of this issue when pulling them from Hackage. This is a pretty minor change, and I'm not 100% sure of the reason why the upstream change was made, so feel free to take it or leave it for your own purposes. Thanks for your time!