os.tmpDir Error due to depreciated

[jny986]

Using Gittio with the latest version of os causes the following error:

`/Users/josh/.config/yarn/global/node_modules/gittio/lib/dist.js:59
return path.join(os.tmpDir(), Date.now().toString() + '-' + Math.random().toString().substring(2));
^

TypeError: os.tmpDir is not a function
at generateTempDir (/Users/josh/.config/yarn/global/node_modules/gittio/lib/dist.js:59:23)
at Object.exports.download (/Users/josh/.config/yarn/global/node_modules/gittio/lib/dist.js:43:27)
at /Users/josh/.config/yarn/global/node_modules/gittio/lib/gittio.js:336:14
at Array.forEach ()
at Function..each..forEach (/Users/josh/.config/yarn/global/node_modules/underscore/underscore.js:79:11)
at component.lookup.action (/Users/josh/.config/yarn/global/node_modules/gittio/lib/gittio.js:320:7)
at Request._callback (/Users/josh/.config/yarn/global/node_modules/gittio/lib/component.js:40:7)
at Request.self.callback (/Users/josh/.config/yarn/global/node_modules/request/request.js:185:22)
at Request.emit (events.js:315:20)
at Request. (/Users/josh/.config/yarn/global/node_modules/request/request.js:1154:10)`

I will try to create a backwards compatible fix for this and submit if I have the chance