Clarify launcher env

[natalieparellano]

Fixes (maybe) #162

[ekcasey]

@natalieparellano I think to fully resolve the issue brought up in #162 we should open a separate PR to the platform spec that reorganized the Environment section.

The Current Organitzation is

• Environment
  • Buildpack Environment
    • Stack-Provided Variables
    • User-Provided Variables
  • Launch Environment

But I think, to cover all of the cases clearly we want something more like:

• Environment
  • Buildpack Environment
    • Stack-Provided Variables
    • User-Provided Variables
    • Buildpack-Provided Variables
  • Launch Environment
    • Stack-Provided Variables
    • User-Provided Variables
    • Buildpack-Provided Variables

[natalieparellano]

@ekcasey I agree with your suggestion. Should we re-point #162 at Buildpack 0.7? We could keep this PR as a small incremental improvement, or just save everything for later. But I fear that keeping #162 in Buildpack 0.6 will delay the 0.6 release longer than we might like.