Erase link of removed Firefox Add-On

Add-on 'FireSass' removed by author.
Merge pull request #1448 from aTei/stable

Erase link of removed Firefox Add-On
adding appveyor configuration to this branch off stable
use a larger build matrix

new versions of Ruby (with DevKit) have been installed:
  - 1.9.3 x86 (C:\Ruby193 - default in PATH)
  - 2.0.0 x86 (C:\Ruby200)
  - 2.0.0 x64 (C:\Ruby200-x64)