Make yesod-persistent build on 7.8.1

I'm not sure *why* this pattern doesn't compile anymore with GHC 7.8, and neither does #haskell. The fact remains, however, that it doesn't. These commits expand the type synonym for `runDBSource`. I've filed GHC ticket #8978 (https://ghc.haskell.org/trac/ghc/ticket/8978) for this issue.