Add gcc 4.9 to Debian Stable using Jessie

We need a newer gcc than ships with Debian stable and the best option I can come up with is to pull 4.9 from Jessie which also means pulling in additional dependencies _including_ a newer libc. This container now builds io.js/v0.12 but I'm wavering because it's almost not Debian stable anymore since we're upgrading core components.

Thoughts & review please @ghostbar & @wblankenship