lessc: update from 1.4.2 to 1.7.5
emacs-magit: fix URL (hopefully)
Disable all Haskell builds for Darwin on Hydra.

Hydra generates a GHC closure for Darwin that for no apparent reason
contains an ancient, broken Haddock binary -- probably because of an
impurity in the build system. That bug makes those GHC binaries
unusable: <https://github.com/NixOS/nixpkgs/issues/2689>.