In general, your favorite method works. Here are some options.

Install pathogen.vim, then copy and paste:

git clone git://github.com/vim-ruby/vim-ruby.git ~/.vim/bundle/vim-ruby

Install Vundle, then add the following to your vimrc:

Bundle 'vim-ruby/vim-ruby'

Wait for an upgrade to Vim and install it. Vim ships with the latest version of vim-ruby at the time of its release. (Remember this when choosing another installation method. The version you download will supersede the version that ships with Vim, so you will now be responsible for keeping it up-to-date.)

Download and extract an archive, and run bin/vim-ruby-install.rb to copy the relevant files to ~/.vim.