For ruby < 2.0, look for a proxy to use

Fixes #23 

For ruby > 2.0, Net::HTTP.new will look for the http_proxy environment variable but nor for older rubies.
This looks explicitly for it.

@miketheman 
I don't have any information about the relative frequency of this and we can remove it if you want. But it doesn't really cost anything to support.
If I look for "proxy env variable" in google, I get [here](https://wiki.archlinux.org/index.php/proxy_settings#Environment_variables) and they set them in lower case so it may be used by some