Add missing package "blake2b" (removed by mistake).

[andreibancioiu]

The dependency was removed by mistake when extracting the walletcore package to a separate repository.

The issue was not detected before, since blake2b was present in node_modules, as erdjs-walletcore is added as a dev-dependency in erdjs (only for integration tests). Furthermore, it seems that browserify (used to prepare the bundles for browser tests) was picking up the dependency from node_modules, as well.

When building using webpack, make sure you include the following fallback, as well:

npm install stream-browserify --save-dev

Then, in webpack's configuration file:

fallback: {
    stream: require.resolve("stream-browserify")
}