Remove warning when bundle install

There are some duplicated dependencies in the r_hapi.gemspec, so whenever you try to bundle install it in a project with Gemfile, the following warning message will appear:
```
r_hapi at /Users/oscar/.rvm/gems/ruby-2.1.3/bundler/gems/rHAPI-b06fafc37fdf 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 curb (~> 0.7.12), (~> 0.7.12) use:
    add_runtime_dependency 'curb', '~> 0.7.12', '~> 0.7.12'
```

This PR remove the duplicates (and fix the warning).