Use OpenSSL::Digest instead of deprecated OpenSSL::Digest::Digest

This gets rid of the "Digest::Digest is deprecated; use Digest" warning when using ruby 2.1
https://github.com/ruby/ruby/blob/ruby_1_8_7/ext/openssl/lib/openssl/digest.rb#L51
https://github.com/ruby/ruby/pull/446

Also cleaned up whitespace and formatting!