build: Update dependencies and fix deprecated messages

- All the dev dependencies were updates in package.json.
- prepublish was replaced by prepare to fix "warn prepublish-on-install As of npm@5, `prepublish` scripts are deprecated" & "warn prepublish-on-install Use `prepare` for build steps and `prepublishOnly` for upload-only."
- node-sass was added in package.json and a require('node-sass') was added to Gruntfile.js since the new version of grunt-sass requires it (see: https://github.com/sindresorhus/grunt-sass/releases/tag/v3.0.0)

Gruntfile.js

@@ -8,6 +8,8 @@
 var path = require('path'),
     getCodeVersion = require('silvermine-serverless-utils/src/get-code-version');
 
+const sass = require('node-sass');
+
 module.exports = function(grunt) {
 
    var DEBUG = !!grunt.option('debug'),
@@ -70,6 +72,7 @@ module.exports = function(grunt) {
 
       sass: {
          options: {
+            implementation: sass,
             sourceMap: DEBUG,
             indentWidth: 3,
             outputStyle: DEBUG ? 'expanded' : 'compressed',