Fixed build errors.

I encountered two problems while building aeson-schema. The first one was that it uses Data.Attoparsec.Number, which is deprecated. I've quick-and-dirty fixed it by requiring aeson < 0.7 in the cabal file. The second problem: GHC seems to have issues inferring the right type for i in CodeGen.hs, line 422. I fixed it by adding an explicit annotation.