Bump haskell-src-exts version

[Chris Warburton]

We're stuck on 1.17.* because 1.18.* introduces new parameters (for
annotation).

It's not worth being generic; we can kill backwards compatibility if we
like (as long as we bump our version number!), but going forward it may
be useful.

This is not a priority unless something else breaks because of it
(e.g. Cabal requiring a single version to be shared by all dependencies)