Maintenance/upgrade js dependencies

[tmobaird]

References #382.

@justin808 here's the second PR for updating the dependencies. Note: This PR does NOT include the update of the react libraries. Apparently the new react, react-router, react-dom versions that were released added a lot of breaking changes. An example is removing PropTypes from the react library and adding it to it's own npm module. I'm not sure the amount of updates this will cause, but I figured it was best to leave out of this PR for size.

Any suggestions about how to tackle this?

This PR includes:

• Updating the node version in the .nvmrc
• Adding yarn to engines in package.json's
• Updating JS dependencies

Also I noticed that npm was still included in the engines section of the package.json's. Any idea if this can be removed?

Feel free to let me know any other suggestions you have!

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This change is 

[justin808]

https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html

@tmobaird I think we should try to upgrade. The codemod should make this painless.

[justin808]

@tmobaird I figure it's best if we do the 15.5 one in its own PR.

[tmobaird]

@justin808 awesome, that sounds good. I'll try to knock that out today! 👍