HG-119: Gzip resources on build step

Moved the `build.js` task to `build-all.js`. The new build task depends on `build-all` and only compresses the static resources if the environment is production.

The reason for that is that we need all the resources prepared, before `gzip`-ing them. Doing this on the fly leaves only the gzipped files without the originals, which we still need in case the client does not support compressed resources.
https://github.com/gulpjs/gulp/blob/master/docs/API.md#async-task-support

docs does not tell ...