Installation of haskell-vim-now on NixOS

Please use the following structure, depending on the type of your contribution.

Bug Fixes

• What was the issue(s)?
  Installation of vim-now-haskell failed on NixOS 17.09
  See haskell-vim-now/issues/284
  .- What was the solution?

The issue could be solved by passing --nix parameter to stack in install.sh script

New Features

PR 285 aims to add --nix support in install.sh