Update istanbul and other dependencies

[goto-bus-stop]

Upgrades istanbul, which dropped support for node.js versions < 8. Object.assign can then be used without a polyfill. While I was at it i upgraded all the other dependencies too. There don't seem to be any breaking changes that affect browserify-istanbul.

The latest istanbul version no longer has a transitive dependency on core-js, which saves several megs in installation size and solves a deprecation warning on install.

This requires a major version bump because of the minimum node.js requirement.

Thanks!

[jimmywarting]

The package seems dead, haven't updated in years...

[goto-bus-stop]

Hi @devongovett, would you be open to transferring this package to the browserify github and npm orgs?