Cask/spectacle: upgrade to 0.8.6.

Change the license to MIT as per @vmsp's suggestion, it's been there
since the beginning:
eczarny/spectacle@813e01ac420a13af4b62d1d0d632e81fd4479732
Merge pull request #6882 from daenney/spectacle-0.8.6

Cask/spectacle: upgrade to 0.8.6.