Replace deprecated isparta-loader with istanbul-instrumenter-loader

[JoelLarson]

This removes the deprecated library as outlined in: preboot/angular-webpack#51

It uses a different module than the one specified in the issue above, but it does not require more dependencies than itself.

[escapedcat]

This was helpful to me and I think it should be merged.
@JoelLarson you might want to modify the comment as well to something like:

  // ISTANBUL LOADER
  // https://github.com/deepsweet/istanbul-instrumenter-loader
  // Instrument JS files with istanbul-lib-instrument for subsequent code coverage reporting
  // Skips node_modules and files that end with .test.js

[escapedcat]

Created a PR here: https://github.com/JoelLarson/angular-webpack/pull/2/files

[phra]

@escapedcat when @JoelLarson will merge your PR i will merge this to into the upstream.