Require the latest version of npm

The npm that ships with 0.10 is 1.2.x and it is riddled with
race conditions.

.travis.yml

@@ -1,18 +1,14 @@
 sudo: false
 language: php
-cache:
-  directories:
-  - node_modules
 php:
 - '5.6'
 - '5.5'
 - '5.4'
 
 before_install:
-  - npm cache clean
+  - npm install -g npm@latest
-  - npm prune
   - npm install -g bower gulp jscs
-  - npm update
+  - npm install
   - pyrus install pear/PHP_CodeSniffer
   - phpenv rehash
 

README.md

@@ -15,6 +15,7 @@ Sage is a WordPress starter theme based on [HTML5 Boilerplate](http://html5boile
 
 * PHP >= 5.4
 * Node.js >= 0.10
+* npm >= 2.1.5
 * gulp (`npm install -g gulp`)
 * Bower (`npm install -g bower`)
 
@@ -71,7 +72,7 @@ Sage uses [gulp](http://gulpjs.com/) as its build system and [Bower](http://bowe
 
 ### Install gulp and Bower
 
-**Unfamiliar with npm? Don't have node installed?** [Download and install node.js](http://nodejs.org/download/) before proceeding.
+Building the theme requires [node.js](http://nodejs.org/download/). We recommend you update to the latest version of npm: `npm install -g npm@latest`.
 
 From the command line:
 

package.json

@@ -23,7 +23,8 @@
     "jscs": "jscs gulpfile.js assets/scripts/*.js"
   },
   "engines": {
-    "node": ">= 0.10.0"
+    "node": ">= 0.10.0",
+    "npm": ">=2.1.5"
   },
   "devDependencies": {
     "asset-builder": "^0.3.5",