0.12.8: Fix error from nested @function

v0.12.7 introduced a compact() polyfill by invalidly nesting it in a function-exists check. This invalid condition relied on a Node Sass bug which fixed in v3.5.0.

Fixes

• Update node-sass dependency to a supported v3.x release (@xzyfer, #87)
• Broken in [email protected] (@xzyfer, #84)