fix infinite loop in bang-bang

If you say "hubot !!!", you can cause bang-bang.coffee to go into an
infinite loop. This is because:

1. The filtering only removes entries that are exactly "!!"
2. The `respond` will match anything that starts with "!!"

Thus, hubot will go into an infinite loop like so:

    user: hubot !!!
    hubot: !!!
    hubot: !!!
    hubot: !!!
    hubot: !!!
    ...
Merge pull request #1032 from wadey/bang-bang-inf-loop

fix infinite loop in bang-bang