Update to use Hakyll 4

As discussed in #2 my conversions to use Hakyll 4 (Version 4.3.3.0 to be precise.)
I just figured this out by looking at other people's questions and notes on how to convert so please review carefully …

Hakyll 4 apparently dropped support for Hamlet (https://github.com/jaspervdj/hakyll/issues/113), so the template was
converted to a default Hakyll one as well.