Compatibility with GHC 7.6.1

This relaxes version constraints in the Cabal package description and fixes references to the formerly deprecated, now nonexistent `Prelude.catch` function, enabling compilation with GHC 7.6.1.

The ongoing incompatibility with cabal-install 1.16 remains unaddressed, so hsenv will still be unusable for most users who update ahead of the Haskell Platform, but hopefully this is a step in the right direction.