update https-proxy-agent to fix vulnerability

[evan-scott-zocdoc]

Fixes https://nodesecurity.io/advisories/593

Looked at the changelog and it doesn't appear there is any breaking change relevant to the library.

https://github.com/TooTallNate/node-https-proxy-agent/blob/master/History.md#200--2017-06-26

[mohitmun]

@evan-scott-zocdoc Did you try running tests locally? travis build is failing since credentials are outdated. we are fixing it soon

[evan-scott-zocdoc]

One of the tests failed for me locally, but the same tests fails for me too on master. So I don't know what's up.

(latest macOS, node 8.11.2)