see if haskellPackages.yaml works on 19.03

[Chris Warburton]

It was broken on 18.09, so we pinned a load of things to old versions.

It looks like it might work on 19.03, in which case either:

 - We don't need to pin those things anymore

Or:

 - They fail for some other reason, so we should be testing that instead
   of yaml