Support Sass 3.5

[HamptonMakes]

I'm not sure why we are stopping using the "latest" version.

[jonathanhefner]

import-once/compass-import-once.gemspec also needs this change, as per #2135. 👍

Also cli/compass.gemspec, I suppose, going by 7cb58273.

[junaruga]

@hcatlin could you update below gemspec files to update not only compass-core but also compass and compass-import-once?

./cli/compass.gemspec
./core/compass-core.gemspec
./import-once/compass-import-once.gemspec

Thank you.

[junaruga]

See this repo's top page.

https://github.com/Compass/compass
Compass is no longer actively maintained.

We should use the compass alternative.