Fix gemspec for rubygems 2.x

After upgrading a project to Ruby 2.1.1 (Rubygems 2.2.2), I started seeing the following when running bundle install:

```bash
cache_fu at .bundle/bundler/gems/cache_fu-06adc686decf 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 rails (>= 2.3, development), (>= 2.3) use:
    add_runtime_dependency 'rails', '>= 2.3', '>= 2.3'
Using cache_fu 0.3.0 from git@github.com:kreetitech/cache_fu.git (at master)
```

This fixes the gemspec in a way that will be compatible with both older and newer versions of Rubygems.