Fix issue #444 (hakyll runs out of file handles)

[BinderDavid]

Fixes #444
On systems which don't provide enough file handles to processes, building the site with hakyll can fail since hakyll holds on to too many file handles at the same time.
This commit bumps the minimum requirement on hakyll to version 4.16.1.0, which fixes the problem.

I am just running this through CI for the moment, to see if it also works there.

[BinderDavid]

Since this hakyll version is just 1 day old on Hackage, people will have to run cabal update in order to be able to build. The error message one gets otherwise is known to not be ideal in that case: haskell/cabal#8922

[BinderDavid]

@david-christiansen
You wrote in the issue:

I suppose we should experiment a bit with bounds to make sure this release works with everything that we want to support, then close this up.

Did you have anything specific in mind which maybe would not work? I also made sure that the site can be built using stack buildagain, so I think we could just bump the lower bound for hakyll in the cabal file. The only issue I can foresee is that people have to update their cabal index with cabal update before they can build the site again, cabal is unfortunately not very helpful in that case.

[david-christiansen]

The only thing I had in mind would be if the new Hakyll required a newer base or some other library, but a quick skim of the dependencies doesn't reveal any obvious issues.