Added 'watch' script and updated JS dependencies

[seniorquico]

This is a tweak to #175.

I've replaced the "dev" script with a "watch" script that uses watchify to continuously output the JS to the TestHost project folder. The proposed development workflow is as follows (in separate terminals):

$ cd App
$ npm install
$ npm run watchify

$ cd TestHost
$ dotnet run

With these running, simply refresh the browser page after saving a .js/.jsx file.

I also updated the JS dependencies. Notably, this now uses Babel 7 and the newer env preset. I'll also point out that the react-chartjs package appears to have issues with React 16. This package will need to stay pinned to React 15 until a fix or alternative is introduced.

[richorama]

Awesome - thanks @seniorquico :¬)

[carloslazo]

Thank you both. Works great!