Export the ENV variables for the entire compilation process.
Resolved Issue OSP-25: export_env_dir for entire compile

The user config variables are only being used for the collectstatic step
 and should be available throughout.

Subcommit(s):
  47187ca Export the ENV variables for the entire compilation process.
Export the ENV variables for the entire compilation process.
Merge branch 'development' of github.com:localmed/heroku-buildpack-python-extras into development
Change env prefixes from DISABLE_ to SKIP_
Update default bundler version
Add more information about which versions are supported
Update pip version
Remove --use-mirrors