Fix invalid gemspec for activeadmin dependency

After updating to ruby 2.1.1 Bundler started complaining about duplicate dependency definitions:
```
The validation message from Rubygems was:
  duplicate dependency on activeadmin (~> 0.4.3, development), (>= 0.4.0) use:
    add_runtime_dependency 'activeadmin', '~> 0.4.3', '>= 0.4.0'
```
This fixes it.