Upgrade extract-text-webpack-plugin to 2.1.2 from 2.1.1

[yingw787]

The problem

Just downloaded the boilerplate and it broke on npm start with extract-text-webpack-plugin, relating to absolute vs. relative path issue noted here: webpack-contrib/extract-text-webpack-plugin#536

Details

Update version number in package.json, reinstall node_modules, and everything works fine. Just a heads up.

[KleoPetroff]

Thank you @yingw787! It's strange that the greenkeeper didn't update the dependency, nor the CI failed. I've manually updated the package to version 2.1.2.