Update Node and npm versions mentioned in CONTRIBUTING.md

CONTRIBUTING.md

@@ -50,14 +50,14 @@ Conventions and squad decisions are kept in the [decisions folder](/decisions).
 
 Backpack is developed using Node, using the following versions:
 
-* `LTS` (Node)
-* `^5.6.0` (npm)
+* `LTS/Carbon` (Node ^8.12.0)
+* `^6.4.1` (npm)
 
 This is enforced using a pre-install hook that calls out to [ensure-node-env](https://github.com/Skyscanner/ensure-node-env).
 
 If you use [nvm](https://github.com/creationix/nvm) or [nave](https://github.com/isaacs/nave) to manage your Node environment, Backpack has built-in support for these. Just run `nvm use` or `nave auto` to install the correct Node version.
 
-To install npm, use `npm install --global npm@^5.6.0`.
+To install npm, use `npm install --global npm@^6.4.1`.
 
 ### React Native