ci: update ghc versions to match pinned nixpkgs

This should fix the travis failures, I think.

One issue with our nix setup is that ghc___Binary versions don't work
because they are treated differently in nixpkgs.

    > GHC_VERSION=ghc865Binary nix-build
    error: attribute 'haskellCompilerName' missing, at /nix/store/21wy3lycrr37p7i3czaz8mvrd576ckqm-source/pkgs/development/haskell-modules/make-package-set.nix:140:32
    (use '--show-trace' to show detailed location information)

For reference, here are all the ghc versions available

    > nix eval '(builtins.attrNames (import (builtins.fetchTarball (import ./nix/nixpkgs.nix)) {}).haskell.compiler)'
    [ "ghc8102Binary" "ghc8102BinaryMinimal" "ghc8104" "ghc865Binary" "ghc884" "ghc901" "ghcHEAD" "integer-simple" "native-bignum" ]

.travis.yml

@@ -2,9 +2,10 @@
   "language": "nix",
   "env": {
     "jobs":
-      [ "GHC_VERSION=ghc844"
-      , "GHC_VERSION=ghc865"
-      , "GHC_VERSION=ghc883"
+      [ "GHC_VERSION=ghc8104"
+      , "GHC_VERSION=ghc884"
+      , "GHC_VERSION=ghc901"
+      , "GHC_VERSION=ghcHEAD"
       ]
   },
   "nix": "2.3.3"

CONTRIBUTING.md

@@ -16,11 +16,11 @@ set the `GHC_VERSION` environment variable:
 GHC_VERSION=ghc881 nix-build
 ```
 
-To find out which GHC versions your nix setup provides:
+To find out which GHC versions our nix setup provides:
 
 ```
-$ nix eval '(builtins.attrNames ((import <nixpkgs> {}).haskell.compiler))'
-[ "ghc822Binary" "ghc844" "ghc863Binary" "ghc865" "ghc881" "ghcHEAD" "ghcjs" "ghcjs86" "integer-simple" ]
+$ nix eval '(builtins.attrNames (import (builtins.fetchTarball (import ./nix/nixpkgs.nix)) {}).haskell.compiler)'
+[ "ghc8102Binary" "ghc8102BinaryMinimal" "ghc8104" "ghc865Binary" "ghc884" "ghc901" "ghcHEAD" "integer-simple" "native-bignum" ]
 ```
 
 # Building your pull request

all-ghcs.nix

@@ -1,5 +1,3 @@
-{ nixpkgs ? import <nixpkgs> {} }:
-
 let
   travisJobEnvAssignments = (builtins.fromJSON (builtins.readFile ./.travis.yml)).env.jobs;
   getGHCVersionFromEnvAssignment = envAssignment: builtins.elemAt (builtins.split "=" envAssignment) 2;