Switch back to proxy-agent

[zkat]

#8 was done because of the massive deps that were getting included with pac-resolver. This was apparently fixed!, which means we can take out the m-f-h-side logic for this and just fall back to proxy-agent again. That should simplify the agent impl a bit 👍

Whoever does this should also comb through the code and make sure proxy-agent is doing everything we need to do over on our end, in case I forgot about something that was added since.

[silverwind]

proxy-agent now supports parsing proxy variables from the environment, so the environment parsing done in module can potentially be removed with such a switch.

[zkat]

@silverwind you wanna PR that? :)

[silverwind]

Maybe later, I see that proxy-agent currently prints a nasty warning on installation, so let's wait until https://github.com/TooTallNate/node-proxy-agent/issues/27 is resolved.