Create export file for subsequent processes

Added a feature to create an "export" file for subsequent processes. "export" file is used in heroku-buildpack-multi.

- https://github.com/ddollar/heroku-buildpack-multi/pull/18

This would be useful when we use ruby packages on build process outside heroku-build-pack. (e.g. compile sass files using compass)

Sample of generated "export" file
```
export GEM_PATH="/tmp/build_d61113c86a29960955eed25246095d1d/vendor/bundle/ruby/2.1.0:$GEM_PATH"
export LANG=${LANG:-en_US.UTF-8}
export PATH="/tmp/build_d61113c86a29960955eed25246095d1d/vendor/bundle/bin:/tmp/build_d61113c86a29960955eed25246095d1d/vendor/bundle/ruby/2.1.0/bin:/tmp/build_d61113c86a29960955eed25246095d1d/vendor/ruby-2.1.0/bin:/tmp/build_d61113c86a29960955eed25246095d1d/:/tmp/codon/vendor/bin:/app/bin:/usr/ruby1.9.2/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/tmp/build_d61113c86a29960955eed25246095d1d/bin:/usr/local/bin:/usr/bin:/bin"
```