Update dependencies

[bjornstar]

• To avoid complaints from npm audit I updated all the dependencies (fixes Update unsecure dependency #69)
• Replaced gulp with simple npm scripts: lint & mocha
• Updated the tested node versions to the three modern versions: 6, 8 & 10
• Updated the README to document the change from gulp to npm scripts
• Replaced istanbul with nyc (it's the same project)
• Updated the minimum node version to 6
• Regenerated docs (there was an issue with one of the links so I tweaked it to match the other links)
• Removed unused, deprecated eslint option: ecmaFeatures

[bjornstar]

Is there anything I can do to get this looked at?

[bjornstar]

Any interest in moving forward with this?

[oliverjessner]

Sadly it looks like the tumblr team is not very active :(

[ceyko]

Hey @bjornstar this looks great, thanks!

Only thing we need from you now to merge this is to accept our CLA. I don't see one currently on file for your github user, so can you please visit https://yahoocla.herokuapp.com/ and agree to the terms? Thanks! :)

[yahoocla]

Thank you for submitting this pull request, however I do not see a valid CLA on file for you. Before we can merge this request please visit https://yahoocla.herokuapp.com/ and agree to the terms. Thanks! 😄

[bjornstar]

Thanks! I just signed the CLA.