Add the Yarn package manager

[nickytonline]

It would be good to have the Yarn package manager used instead of npm install. The main reason for proposing this is pretty much why Yarn was built, fast and reliable dependency management. For those not aware of Yarn, you can read more about it here, https://yarnpkg.com.

I will put up a PR shortly for this.

[coryhouse]

Closed via PR #292

[nickytonline]

Thanks for merging @coryhouse. Here's a screen shot of Travis CI for react-slingshot. As you can see there's a big difference in build times now.

[coryhouse]

Great improvement! :)