Fix undefined var: "ReferenceError: imgur is not defined"

[jwflory]

This is a bug where the existing code in master branch does not work because of an undefined variable:

$ docker run --rm -it teleirc-fedora-tmp
node-telegram-bot-api deprecated Automatic enabling of cancellation of promises is deprecated.
In the future, you will have to enable it yourself.
See https://github.com/yagop/node-telegram-bot-api/issues/319. module.js:653:30
Reading config file to get bot API token...
/opt/teleirc/lib/TeleIrc.js:39
      imgur.setClientId(this.config.imgur.imgurClientId);
      ^

ReferenceError: imgur is not defined
    at new TeleIrc (/opt/teleirc/lib/TeleIrc.js:39:7)
    at Object.<anonymous> (/opt/teleirc/teleirc.js:12:15)
    at Module._compile (module.js:653:30)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Function.Module.runMain (module.js:694:10)
    at startup (bootstrap_node.js:204:16)
    at bootstrap_node.js:625:3

[jwflory]

Addressed in #90.