Remove use of `superagent-proxy`

[tommoor]

Hey @jonkemp thanks for this useful module. Unfortunately the module depends on vm2 which has critical security vulnerabilities and is no longer maintained.

Fortunately the vm2 dependency is introduced through the use of superagent-proxy which according to the author doesn't really have a reason to exist anymore.

I suggest that superagent-proxy should also drop the parameter in .proxy(), and simply apply the ProxyAgent instance. Typing this out, I question whether this module even needs to exist anymore, since it's essentially would just boil down to:

I think we could remediate the security issue by removing this package with a small patch