Update eslint and related packages; fix new linting errors

[realityking]

The most significant change is that tests now use strict assertions, legacy assertions were deprecated with Node.js 10.

[simov]

Thanks, I saw your other PR too. Tests are failing on Node 6 due to missing assert.strict.

[realityking]

Oi that's annoying. Two options:

1. Drop Node 6 from TravisCI
2. Disable node/no-deprecated-api in test.js

Which do you prefer?

[simov]

I prefer having the LTS versions for a bit longer, especially while they are still supported. No idea what node/no-deprecated-api would do, is it an ESLint rule?

[realityking]

node/no-deprecated-api is the eslint rule that gets triggered. I pushed a version that disables that rule for the test file.