abis.json is not optimized, and we are bundling all the invididual abis as well

I merged https://github.com/daostack/arc.js/pull/460/files a bit too soon, there are two issues with this release:

* `abis.json` is very big (927K), because it duplicates all the JSON files. We should use teh `rootVersion` optimization when generating the file.
* we are now bundling BOTH all the invidual ABI files, as well as the abis.json file where all these files are merged in. Of course, `dist/lib/abis` should only contain the `abi.json` file.