Hashie version dependency is tight

The gemspec uses  'hashie', '~> 3.4.2' but another gem I use requires hashie >= 3.5.2. I don't see significant [changes from 3.4.2 to 3.5.0](https://github.com/intridea/hashie/blob/master/CHANGELOG.md) so I plan to try loosening the dependency and test it out