Revert to earlier version of babel-minify-webpack-plugin due to sourcemap errors

[eliperelman]

Fixes #578.

This is a workaround for webpack-contrib/babel-minify-webpack-plugin#68 until that issue is fixed. Right now we are pinning to a previous version.

[helfi92]

If we're no longer using babel-minify, should the package name be changed to something else?

[eliperelman]

We are just using an older version of it, we are still using it.

[helfi92]

Sounds good.

[eliperelman]

And the name of the package is a reference to what we are doing, rather than what it contains. We are doing a babel minify, just like image-minify minifies images, rather than reference the imagemin plugin.