Relax cabal requirements for Heroku build

With these changes the Anvil build *almost* passes. I think the Heroku environmental factors are good now and the only problem is updating this project to use new Wai Websockets stuff. Here is the error I get at the end of a mostly successful Anvil build:

```sh
Building graph-rtce-0.0.0.0...
Preprocessing executable 'graph-rtce' for graph-rtce-0.0.0.0...
[1 of 3] Compiling Pages.Index      ( src/Pages/Index.hs, dist/build/graph-rtce/graph-rtce-tmp/Pages/Index.o )

src/Pages/Index.hs:8:1: Warning:
    Top-level binding with no type signature:
   render :: blaze-markup-0.6.1.0:Text.Blaze.Internal.MarkupM ()
[2 of 3] Compiling Types            ( src/Types.hs, dist/build/graph-rtce/graph-rtce-tmp/Types.o )
[3 of 3] Compiling Main             ( src/Server.hs, dist/build/graph-rtce/graph-rtce-tmp/Main.o )

src/Server.hs:18:40:
    Module ‘Network.Wai.Handler.WebSockets’ does not export ‘intercept’
```