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Those are outdated atm and use old Buffer API, which is going to start printing runtime warnings on Node.js 10.0.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

[shibukawa]

And security issues exist:

• https://snyk.io/vuln/npm:http-proxy-agent:20180406
• https://snyk.io/vuln/npm:https-proxy-agent:20180402

[ChALkeR]

@shibukawa That was the primary reason for opening this issue here.

[ChALkeR]

@TooTallNate Thanks!

[shibukawa]

@TooTallNate Thank you for your quick response!