Fix deprecation

[nodeg]

I see the following deprecation warning when using Gitarro in our Jenkins CI:

Top level ::CompositeIO is deprecated, require 'multipart/post' and use `Multipart::Post::CompositeReadIO` instead!
Top level ::Parts is deprecated, require 'multipart/post' and use `Multipart::Post::Parts` instead!
/usr/lib64/ruby/gems/2.5.0/gems/faraday-0.17.3/lib/faraday/upload_io.rb:65: warning: constant ::UploadIO is deprecated
/usr/lib64/ruby/gems/2.5.0/gems/faraday-0.17.3/lib/faraday/upload_io.rb:66: warning: constant ::Parts is deprecated

Maybe a simple bump of the required Gems can already fix that. At some point, however, a bump of the minimal Ruby version would be nice.