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)
Merge pull request #133 from sp4ke/master

Use serve-static@1.7.1