Add ruby-version, require rubygems

Just a hunch, but try this: 

I've got it successfully running on osx after making sure I had the right symlinks in place - are you trying to run this on windows?

One more thing to note - in order to get all of the dependency extensions built, I had to run

```
sudo ln -s /usr/local/Cellar/libffi/3.0.13/lib/pkgconfig/libffi.pc /usr/lib/pkgconfig
sudo ln -s /usr/local/Cellar/libxml2/2.9.1/lib/pkgconfig/libxml-2.0.pc /usr/lib/pkgconfig
```

What are you using for ruby versioning? This is also highly recommended as opposed to using system rubies