Clamp hubot-irc dependency to 0.0.8.

The hubot-irc readme at nandub/hubot-irc says this:

> This bot is now upgraded to Hubot 2.3. You should specify 0.0.8 as your
> hubot-irc dependency for older versions of Hubot (<2.3.0).

Since we're using hubot 2.1.4, we should specify hubot-irc 0.0.8. Otherwise
running hubot with the irc adapter throws a funky "Cannot read property
'prototype' of undefined" error on startup.