remove duplicate httparty dependency

otherwise, bundler complains:

```
helpscout at /Users/visnup/Development/web/vendor/bundle/bundler/gems/helpscout-4ed91bb6f237 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 httparty (>= 0, development), (>= 0) use:
    add_runtime_dependency 'httparty', '>= 0', '>= 0'
```