Pass --reorder-goals to "cabal install"...

Tweaks the cabal dependency resolution heuristics in some undocumented way that makes cabal more likely to solve Yesod dependencies.  (Without this you see odd errors like yesod-platform getting rejected because it depends on a conflicting version of, say, aeson, than your app does, even though your .cabal file doesn't actually specify which version of aeson, and the whole point of yesod-platform was to lock it to a version that would work.)

This feels like black magic, but it's [recommended by the Yesod documentation](http://www.yesodweb.com/page/quickstart) :(