Upgrade webpack to 2.x or 3.x (if 3.x looks stable enough)

Webpack 2/3 is needed for recent releases of storybook, so we should upgrade the version used for the main build so that we are not maintaining different versions of webpack config.

Webpack 2->3 only has changes to the plugin API, not the config API. We should only fall back on Webpack 2 if our plugins don't work with webpack 3.