Don't allow sass 3.4+ in the gemspec

This is tied to issues mentioned in:
http://foundation.zurb.com/forum/posts/18856-sass-342-compilation-proble
m
https://github.com/sass/sass/issues/1397
https://github.com/zurb/foundation/issues/5636
https://github.com/zurb/foundation/pull/5632

Since it seems to be taking time for smarter minds to come up with
changes to Foundation 5 that will get it playing nice with sass 3.4,
let’s at least modify the dependencies in the meantime so more users
don’t update their gems and spend forever debugging a very unusual
issue.