add happy dep to list of cabal installs

`haskell-src-exts` is required by `ghc-mod` and a number of the other packages.  `haskell-src-exts` requires that `happy` be installed but for some reason it is not listed as a dep.  With this change this `cabal install` command works on a fresh install.
Merge pull request #11 from bmabey/patch-1

add happy dep to list of cabal installs