Simplify sass cache rules

`.sass-cache` is the cache directory created by the [Sass](http://sass-lang.org/) CSS preprocessor.

  * [Compass](http://compass-style.org/) extends Sass, and I would think that most people who use it know this, so it seems unnecessary to have a duplicate single-rule template for it.
  * Rails [redirects the Sass cache to `tmp/sass-cache`](http://sass-lang.com/documentation/file.SASS_REFERENCE.html#caching), so it's handled by the existing rule for `/tmp`.
  * The Sass template was under Global/, but I think that whenever you have Sass files in your repository, you're going to want the rule in its gitignore file anyway. I don't feel strongly about this.