Added http_proxy environment variable to the geminabox init script

Prior to rubygems.org switching to the new fastly.net CDN the http proxy configuration appeared to be working... Now I must export the http_proxy environment variable in the init script to make the proxy_url functional for httpclient:

* added http_proxy environment variable to geminabox init script erb template
* added subscribe to geminabox service for changed made to the init script