remove unused Gruntfile dependencies (#471)

no issue
- many of our grunt tasks have already been removed but we still had the imports and respective npm dependencies lying around

ghost/admin/Gruntfile.js

@@ -1,12 +1,5 @@
 /* eslint-env node */
-/* eslint-disable no-var, one-var, object-shorthand, prefer-template */
-var _              = require('lodash'),
-    fs             = require('fs-extra'),
-    path           = require('path'),
-    https          = require('https'),
-    moment         = require('moment'),
-    chalk          = require('chalk'),
-    Promise        = require('bluebird');
+/* eslint-disable object-shorthand */
 
 module.exports = function(grunt) {
 

ghost/admin/package.json

@@ -25,7 +25,6 @@
     "node": "~0.12.0 || ^4.2.0"
   },
   "devDependencies": {
-    "bluebird": "3.4.6",
     "blueimp-md5": "2.6.0",
     "bower": "1.8.0",
     "broccoli-asset-rev": "2.5.0",
@@ -33,7 +32,6 @@
     "broccoli-concat": "3.0.5",
     "broccoli-merge-trees": "1.2.1",
     "broccoli-uglify-js": "0.2.0",
-    "chalk": "1.1.3",
     "codemirror": "5.22.0",
     "csscomb": "3.1.8",
     "ember-ajax": "2.5.3",
@@ -84,7 +82,6 @@
     "liquid-fire": "0.27.0",
     "liquid-wormhole": "2.0.2",
     "loader.js": "4.1.0",
-    "lodash": "4.17.2",
     "matchdep": "1.0.1",
     "moment": "2.17.1",
     "moment-timezone": "0.5.11",