Fix glob caching bug

Needed to ensure our "virtual" glob sass file has a unique name otherwise it gets `.uniq`'d alway in Sass's dependency `Set` here https://github.com/sass/sass/blob/2bdc564dea068c4e19064daea25349d93f8d01b8/lib/sass/engine.rb#L331

It sorta makes me worry a little bit about the virtual file approach. `:filename => nil` would make more sense in this case, but other internal sass APIs seem to assume `:filename` exists for any relative import calls. So those would crash or be incorrect if we set it to `nil` or some random value. Another approach might be writing out a "real" file to tmp, but I don't know how that would affect sass's internal cache.

Fixes #299.

/cc 
  @ksylvest if you wouldn't mind testing this branch before we do a final release
  @rafaelfranca we'll probably want to cut a new patch level release after this.