Allow Hashie 3 or lower

I use a gem that requires Hashie > 3.0 and it is causing a gem conflict with RestForce.

I upgraded the dependency to allow for 3.0 and ran the specs and they run clean. I also when through the list 
https://github.com/intridea/hashie/blob/master/UPGRADING.md and RestForce does not seem to be affected by any of the Hashie 3 changes.