Removed duplicate ffi dependency

This didn't make much sense.. but I detected it when I saw this in my `Gemfile.lock` (which is no doubt Bundler's issue but it made me look at the dependencies you had!):

```
    levenshtein-ffi (1.0.3)
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
      ffi (~> 1.1.5)
```