Bump version of NodeJS to 10.15.3

Updates the README to reference this.

If running this locally, it's recommended to use
the 'n' NPM package to manage your version of
NodeJS, as described in the README:

https://github.com/alphagov/notifications-admin#first-time-setup

README.md

@@ -22,19 +22,19 @@ Brew is a package manager for OSX. The following command installs brew:
 
 Languages needed
 - Python 3.4
-- [Node](https://nodejs.org/) 5.0.0 or greater
-- [npm](https://www.npmjs.com/) 3.0.0 or greater
+- [Node](https://nodejs.org/) 10.15.3 or greater
+- [npm](https://www.npmjs.com/) 6.4.1 or greater
 ```shell
     brew install node
 ```
 
 
 [NPM](npmjs.org) is Node's package management tool. `n` is a tool for managing
-different versions of Node. The following installs `n` and uses the latest
+different versions of Node. The following installs `n` and uses the long term support (LTS)
 version of Node.
 ```shell
     npm install -g n
-    n latest
+    n lts
     npm rebuild node-sass
 ```
 

package.json

@@ -3,7 +3,7 @@
   "version": "0.0.1",
   "description": "Admin front end for GOV.UK Notify",
   "engines": {
-    "node": "5.0.0"
+    "node": "10.15.3"
   },
   "scripts": {
     "test": "gulp lint",