[closebracket addon] Fix: Trigger auto-close-triple behaviour after exactly 3 quotes.

Issue #2385

I have updated the condition for auto-close-triple behaviour.

![test1](https://cloud.githubusercontent.com/assets/5851313/2574522/25c7b232-b935-11e3-8514-5e96f4a2fcb8.gif)

Tested in Chromium 33.0.1750.152 Ubuntu 12.04 (256984)