Loosen restrictions on hashie gem.
Merge branch 'dan/hashie' into dan/master
Merge branch 'dan/faraday-0.9' into dan/master
Add a link to ActiveForce on the readme.

Link to ActiveForce, a gem based on a [gist from @ejholmes](https://gist.github.com/ejholmes/5154415) that wrap Restforce in an ActiveRecord like functionality.
Merge pull request #123 from crisply/dan/hashie

Loosen restrictions on hashie gem to < 4.0