[MODEL] Removed duplicate "activesupport" dependency from elasticsearch-model.gemspec

When installing elasticsearch-model from the git repository, Bundler says:

```
elasticsearch-model at /home/nono/.gem/ruby/2.1.0/bundler/gems/elasticsearch-rails-fc966380167b/elasticsearch-model 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 activesupport (> 3.0, development), (> 3) use:
    add_runtime_dependency 'activesupport', '> 3.0', '> 3'
```

Closes #14