Restore compat with GHC 7.4
Restore compat with GHC 7.4

Without this fix, GHC 7.4.2 fails install `criterion` with:

app/Options.hs:13:8:
    Could not find module `GHC.Generics'
    It is a member of the hidden package `ghc-prim'.
    Perhaps you need to add `ghc-prim' to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.

bos/criterion,iu-parfunc/criterion,bgamari/criterion,iu-parfunc/criterion,bos/criterion,bgamari/criterion,bos/criterion,iu-parfunc/criterion,bgamari/criterion