remove ffi dependency

This somehow snuck into my lockfile from #259: https://github.com/bacongobbler/deis/commit/1ca8d840f5b655a269370aa9d7a8fe8e7c8da577#diff-e79a60dc6b85309ae70a6ea8261eaf95R70

There's no need for it, I think. knife-digital_ocean does not depend on this gem. People are running into this issue:

```
Could not find ffi-1.9.1 in any of the sources
```