Recover if proxy to rubygems is unreachable

The pass through proxy to rubygems is awesome, but I assumed that if
rubygems was down for some reason the geminabox server would still work.
Currently this isn't the case

    % bundle
    Fetching source index from http://mygeminaboxserver.com/
    Retrying source fetch due to error (2/3): Bundler::HTTPError Could not fetch specs from http://mygeminaboxserver.com/
    Retrying source fetch due to error (3/3): Bundler::HTTPError Could not fetch specs from http://mygeminaboxserver.com/
    Could not fetch specs from http://mygeminaboxserver.com/

On the server side:

    % rackup
    [2014-02-17 12:32:34] INFO  WEBrick 1.3.1
    [2014-02-17 12:32:34] INFO  ruby 1.9.3 (2013-11-22) [x86_64-darwin13.0.2]
    [2014-02-17 12:32:34] INFO  WEBrick::HTTPServer#start: pid=52051 port=9292
    127.0.0.1 - - [17/Feb/2014 12:32:48] "GET /api/v1/dependencies HTTP/1.1" 200 - 0.0221
    SocketError - getaddrinfo: nodename nor servname provided, or not known (http://rubygems.org:80):
    ...
    httpclient-2.3.4.1/lib/httpclient.rb:594:in `get_content'
    (eval):4:in `get_content'
    geminabox-0.12.2/lib/geminabox/proxy/file_handler.rb:46:in `remote_content'
    geminabox-0.12.2/lib/geminabox/proxy/splicer.rb:49:in `merge_gziped_content'
    geminabox-0.12.2/lib/geminabox/proxy/splicer.rb:37:in `merge_content'
    geminabox-0.12.2/lib/geminabox/proxy/splicer.rb:17:in `new_content'
    geminabox-0.12.2/lib/geminabox/proxy/splicer.rb:12:in `block in create'
    geminabox-0.12.2/lib/geminabox/proxy/splicer.rb:12:in `open'
    geminabox-0.12.2/lib/geminabox/proxy/splicer.rb:12:in `create'
    geminabox-0.12.2/lib/geminabox/proxy/splicer.rb:7:in `make'
    geminabox-0.12.2/lib/geminabox/proxy/hostess.rb:72:in `splice_file'
    geminabox-0.12.2/lib/geminabox/proxy/hostess.rb:22:in `block (2 levels) in <class:Hostess>'
Fix http_proxy env variable useage

Not sure how the person submitting the patch to use the http_proxy
environment variable every had things working because

    irb(main):001:0> ENV['http_proxy'].empty?
    NoMethodError: undefined method `empty?' for nil:NilClass