Flag +ruby means that vim is compiled with ruby support and linked against ruby interpreter library. You cannot get this flag without recompiling vim or installing another version which is compiled with this flag.
If there isn't a Ruby enabled Vim available for your operating system, you'll have to recompile. This is very easy and there are some instructions on the Vim website I believe. If you're on Linux, you use configure to choose what you want. Have a look at the output of:
./configure --help
in the Vim source directory. Read it carefully, there are a lot of options in there. The main one you want is --enable-rubyinterp, but you may also want to add --with-features=HUGE among others. As I said, read the help provided.
If you are lazzy and don't want to recompile you can try to find a package with a vim version including ruby. On debian it's vim-ruby so something like
apt-get install vim-ruby
might work. (I can't try it, I m on mac. On mac , MacVim come with ruby enabled)
Pulling the vim source using Mercurial and changing into the directory will give you the ability to configure your vim install before you compile it.
hg clone https://vim.googlecode.com/hg/ vim
cd vim
./configure --enable-rubyinterp
The --enable-xxinterp option can be used for Python, Perl, or any other language that Vim will support. Just type it in where the xx is and it will work.
Running the help option with the configure command will allow you to see all of the configuration options.
On Mac OS X, I find that the easiest is to install MacVim with brew install macvim which includes +ruby. And then symlink /usr/local/bin/vim to /usr/local/bin/mvim. That way to get a recent Vim version, with the huge feature set, +ruby, both GUI and command line vim just using the standard HomeBrew repository. No need for external repository like in Pierre answer
To avoid issues it's better to use the use the system ruby during installation so:
rvm use system
brew install macvim
ln -s /usr/local/bin/mvim /usr/local/bin/vim