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 think it's a legacy libcurl thing. `man curl`:

```
ENVIRONMENT
       The environment variables can be specified in lower case or upper case. The lower case version has precedence. http_proxy is an exception as it is only available in lower case.

       http_proxy [protocol://]<host>[:port]
              Sets the proxy server to use for HTTP.

       HTTPS_PROXY [protocol://]<host>[:port]
              Sets the proxy server to use for HTTPS.

       FTP_PROXY [protocol://]<host>[:port]
              Sets the proxy server to use for FTP.

       ALL_PROXY [protocol://]<host>[:port]
              Sets the proxy server to use if no protocol-specific proxy is set.

       NO_PROXY <comma-separated list of hosts>
              list of host names that shouldn't go through any proxy. If set to a asterisk '*' only, it matches all hosts.
```