Support proxy servers

Many of the Linux utilities support environment variable `http_proxy` to connect via a proxy server. This idiom is also supported by Capistrano with configuration settings `default_environment['http_proxy']` and `default_environment['https_proxy']`. With these settings enabled deployment environment contains variables `http_proxy` and `https_proxy` which this pull request takes advantage of.

Perhaps it would be elegant to be able to provide proxy settings for `Squash::Ruby.configure`, but I wanted to keep this proposition as simple as possible.