Add new Ruby Gem dependency ports for upcoming Mastodon v2.3.1

multimedia/rubygem-streamio-ffmpeg
WWW: http://github.com/streamio/streamio-ffmpeg

security/rubygem-devise_pam_authenticatable2
WWW: http://github.com/devkral/devise_pam_authenticatable2

security/rubygem-omniauth-cas
WWW: https://github.com/dlindahl/omniauth-cas

security/rubygem-rpam2
WWW: http://github.com/devkral/rpam2