Use serve-static@1.7.1

serve-static 1.6.1 creates the following error when launching a dev server:

	/home/spike/.nvm/v0.11.14/lib/node_modules/ionic/node_modules/serve-static/node_modules/send/node_modules/etag/index.js:55
	    throw new TypeError('argument entity must be string, Buffer, or fs.Stats')
		  ^
	TypeError: argument entity must be string, Buffer, or fs.Stats
	    at etag (/home/spike/.nvm/v0.11.14/lib/node_modules/ionic/node_modules/serve-static/node_modules/send/node_modules/etag/index.js:
	55:11)
	    at SendStream.setHeader (/home/spike/.nvm/v0.11.14/lib/node_modules/ionic/node_modules/serve-static/node_modules/send/index.js:72
	4:15)
	    at SendStream.send (/home/spike/.nvm/v0.11.14/lib/node_modules/ionic/node_modules/serve-static/node_modules/send/index.js:500:8)
	    at onstat (/home/spike/.nvm/v0.11.14/lib/node_modules/ionic/node_modules/serve-static/node_modules/send/index.js:585:10)
	    at Object.oncomplete (fs.js:93:15)

server-static@1.7.1 works fine.

Config:
Ubuntu 14.04 - x64
nvm 0.11.14
ionic 1.2.8