This is currently a recreational project for me to learn more about graph algorithms and practice with FFI in advance of my upcoming ATLRUG talk. Do not expect it to be feature-complete. If you are interested in higher-level bindings to the igraph, you may want to check out the Python or R packages in the igraph documentation. There do not appear to be maintained Ruby bindings compatible with the current release (0.6).
Steffi expects igraph 0.6+. To build igraph from source, download it and
./configure make make install
On OSX, you can also install through homebrew
brew install igraph