Ruby mogilefs-client 3.12.0

Minor cleanups, mainly to fix indentation warnings in Ruby 2.6+

7 changes since v3.11.1 (2017-03-23):

      pool: simplify empty class definition
      new_file/stream: remove unused variable
      mog: add updateclass command
      socket/pure_ruby: favor String#clear if available
      fix indentation warnings with ruby trunk
      doc: https:// URLs instead of git://
      update URLs to point to wiki