Update http-proxy-agent and https-proxy-agent deps

[ChALkeR]

Those are outdated atm and use old Buffer API, which is going to start printing runtime warnings on Node.js 10.0.

Tracking: nodejs/node#19079

[ChALkeR]

Could be blocked by TooTallNate/node-pac-proxy-agent#16

[miso-belica]

@TooTallNate Can you please release new version with updated dependencies? I need the TooTallNate/proxy-agents#43 from HTTPS proxy agent and discovered it's not possible to upgrade. I can help with upgrade if you are OK with it and can advise me what is needed to upgrade the package.

[benleen]

It would be great if someone could take the time to upgrade indeed:
https://snyk.io/vuln/npm:http-proxy-agent:20180406
https://snyk.io/vuln/npm:https-proxy-agent:20180402

[ChALkeR]

@TooTallNate Thanks!

[TooTallNate]

np, sorry for the delay gang.

[ggoodman]

This release appears to have broken all node@4.x users. See #18.