Updates mustache dependency in gemspec

Hello,

I just pushed [mustache v0.99.6](https://rubygems.org/gems/mustache/versions/0.99.6) which includes a plethora of bugfixes, especially concerning Ruby 1.9.3+.
I'm not quite sure how to specify the version properly, but it would be beneficial to depend on `v0.99.6`. This is the last `0.99` release, since I'm aiming for `v1.0.0` next.