Update nix.md to reflect current version (travis-ci#1723)

* Update nix.md to reflect current version

The old doc indicated that only Nix 1.11.2 was installed, when in fact the current version from https://nixos.org/nix/install is used by the [travis-build](https://github.com/travis-ci/travis-build/blob/master/lib/travis/build/script/nix.rb#L43) `nix.rb` script.

* nix: add nix-shell to list of tools

Author: badmutex

user/languages/nix.md

@@ -32,15 +32,21 @@ The default channel for `nixpkgs` will be `nixpkgs-unstable`.
 
 The following command line tools are available in the Nix environment:
 
-- nix-env
+- nix
 - nix-build
+- nix-channel
+- nix-collect-garbage
+- nix-copy-closure
+- nix-daemon
+- nix-env
+- nix-instantiate
+- nix-prefetch-url
 - nix-shell
 - nix-store
-- nix-channel
 
 ## Default Nix Version
 
-Currently, only version "1.11.2" of Nix is provided. In the future, it may be possible to configure different versions with `.travis.yml`.
+This installs the current version of Nix using https://nixos.org/nix/install. In the future, it may be possible to configure different versions with `.travis.yml`.
 
 ## Default Target