remove duplicate dependency in gemspec

This fixes the warning message:

```
cupsffi at ~/.gem/ruby/2.1.2/bundler/gems/cupsffi-5930b0fb8f44 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on ffi (>= 0), (>= 0) use:
    add_runtime_dependency 'ffi', '>= 0', '>= 0'
```

Thanks
K