fix to retrieve environment config for socket.io use case

[heyweswu]

Problem:

as of https://github.com/thoov/ember-websockets/releases/tag/8.0.0 if setting socketIO to true, the addon doesn't import socket.io-client and the resulting error is described here in #172

PR details

I'm proposing a change to properly retrieve the values from config/environment.js so that when setting the block below, socket.io-client gets properly imported

'ember-websockets': {
  'socketIO': true
}

Closes #172

[heyweswu]

@thoov are you still maintaining this repo? if so, i'll also throw up a PR to bump mock-socket to the latest