use URI#find_proxy for proxy detection from env

Faraday rolls [its own proxy detection](https://github.com/lostisland/faraday/blob/df9d5c31c577e6bab53af3e4a6635bb8a738e863/lib/faraday/connection.rb#L83) which doesn't behave comprehensively, not taking into account  `no_proxy` or `https_proxy` environment variables. Net::HTTP [relies](https://github.com/ruby/ruby/blob/106075eac89ede7bcd71616ba78f1344892140a4/lib/net/http.rb#L1028) on [URI#find_proxy](http://ruby-doc.org/stdlib-2.1.2/libdoc/uri/rdoc/URI/Generic.html#method-i-find_proxy). This commits makes Faraday do the same.