Make Webpack and its loaders non-dev dependencies

Build-packs used by Packager.io run `npm install` with the
`--production` flag.

These are the minimum dependencies necessary for Webpack to build its
bundle prior to Sprockets assets precompilation.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #2110 from opf/fix/packager-fix-webpack-build

Make Webpack and its loaders non-dev dependencies