Ruby 2.1

Update the gem to work with ruby 2.1 as well as ruby 1.9.3.

* Fixes deprecation warnings in the Rakefile with gem packaging tasks & rdoc tasks
* Solves a multibyte error under ruby 2.1 by forcing `authentication.rb` to load as ASCII, not UTF8
* Squelch an OpenSSL deprecation warning about `OpenSSL::Digest::Digest` now being just `OpenSSL::Digest`

`rake test` passes locally on 1.9.3-p194 and 2.1.2