tmuxinator/tmuxinator: Manage complex tmux sessions easily

Some users have reported issues when installing via Homebrew, so the RubyGems installation is preferred until these are resolved. tmuxinator aims to be compatible with the currently maintained versions of Ruby. Some operating systems may provide an unsupported version of Ruby as their "system ruby". In these cases, users should use RVM or rbenv to install a supported Ruby version and use that version's gem binary to install tmuxinator.