Replace unmaintained dependencies

[msrose]

Hi, first off just want to say thanks for this library, I've been enjoying using Ghost Inspector for the last year or so.

This package depends on a couple of packages that are no longer maintained, i.e. request and request-promise-native: https://github.com/ghost-inspector/node-ghost-inspector/blob/stable/package.json#L31-L32, and as a result some vulnerability alerts are creeping up in repos that depend on ghost-inspector. It would be good to replace them with something actively maintained, like axios or isomorphic-fetch, to make it easier to patch any vulnerabilities that come up in dependencies.