Yesod updates for GHC 7.8

Same idea as https://github.com/TechEmpower/FrameworkBenchmarks/pull/832. Pinging @kazu-yamamoto.
Does the new version of cabal wrap `install`, `configure`, and `build` into the `install` directive now?

The latest travis run suggests that `start` isn't exiting (30+ minutes in... I wouldn't expect it to take this long in practice).