Bump to fay 0.16

I'll be uploading fay 0.16 shortly. The two changes I see that are needed in yesod-fay are renaming Language.Fay.FFI -> Fay.FFI and removing `configNaked` (declarations are at the top level by default now).

I only grepped and fixed these things, and made sure the package compiles. Let me know if there are any issues!

Unrelated to fay, there was also a compiler warning which I left in there since I'm not sure if it's kept for legacy support:
```
Yesod/Fay.hs:111:1:
    Warning: In the use of `jsonToRepJson'
             (imported from Yesod.Core, but defined in Yesod.Core.Json):
             Deprecated: "Use returnJson instead"
```

Cheers!