Remove nodejs-6_x which is about to enter EOL

- Remove nodejs-6_x
- Set nodejs / nodejs-slim to nodejs-8_x / nodejs-slim-8_x
- Re-generate node2nix generated files using nodejs-8_x instead

pkgs/misc/base16-builder/generate.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env nix-shell
+#! nix-shell -i bash -p nodePackages.node2nix
+exec node2nix -8 \
+     --input node-packages.json \
+     --output node-packages-generated.nix \
+     --composition node-packages.nix \
+     --node-env ./../../development/node-packages/node-env.nix \