chore: updating unproblematic dependencies

[martinheidegger]

• readable-stream updated to @3 doesn't contain changes related to the only use I found.
• The changes in eth-rpc-errors does not affect the usage.
• The breaking changes of ws in version 8 were a bit too plentiful. Which is why ws is now only at v7, jumping over v6. Both releases are mostly maintenance releases with the Node version now requiring Node 6 (which is a requirement for other libraries too).
• Same goes for the breaking changes of ethereumjs-block (called @ethereumjs/block from v3) which is why its only updated to version 2 in this PR.

The dev dependencies, browserify, eslint and tape have both worked fine after the upgrade. This PR also removes the ethjs dependency as it is unused.

[mcmire]

Hi @martinheidegger, sorry for the delay in replying. This looks good to me — would you mind rebasing this? The checks on this need to be restarted and I can't seem to push to your branch.

[mcmire]

Thank you for this pull request, but we are actively phasing out this package in favor of json-rpc-engine and its related packages. I'm going to close this, but if any of these upgrades are necessary to resolve any security issues, then feel free to submit a new PR with just those upgrades.