Add http_proxy support to the download_cache

We have some users that run CF behind a firewall that prohibits all outgoing connections to the internet, except via a central proxy server. Most buildpacks use curl to download additional staging assets, so setting up $http_proxy inside the container is sufficient to make them work.  The java-buildpack however uses Net::HTTP, which only gets http_proxy support in Ruby 2.0.

Submitted under the ActiveState CLA.