Update gem dependency

Allow hashie 2.1.x. 

Change log
https://github.com/intridea/hashie/blob/master/CHANGELOG.md

Diff
https://github.com/intridea/hashie/compare/v2.0.5...v2.1.2

hashie v2.2 (It hasn't been released yet, though) and v3.0.0 have inconvertibility.
https://github.com/intridea/hashie/blob/master/UPGRADING.md