update nixpkgs to a3a5e2d31ded105878b3bce94f13d8e0d4115f8c
revert to 7.6.1
~
use ghc 7.8.2
adjust packages for ghc 7.8.2
update nixpkgs to 3e9f0bbe56f9e76a3f5b3eced29964bb3b54bfbf