Update happstack-server transformers dependency

[Roel van Dijk]

Hello,

happstack-server-7.0.0 depends on transformers < 0.3. Two weeks a go
transformers-0.3 was released [1]. I've attached a small patch which
bumps the transformers dependency to < 0.4.

I have tested this change by building most happstack packages with
cabal-dev under GHC-7.4.1. There where no errors introduced by this
change.

Regards,
Roel

1 - http://hackage.haskell.org/package/transformers-0.3.0.0