Add runtime dependency hashie to gemspec

bank_job [require hashie/mash](https://github.com/kkosuge/bank_job/blob/master/lib/bank_job/configuration.rb#L1) and use `Hashie::Mash` in these lines

- https://github.com/kkosuge/bank_job/blob/master/lib/bank_job/strategies/bank_job_smbc.rb#L24
- https://github.com/kkosuge/bank_job/blob/master/lib/bank_job/strategies/bank_job_smbc.rb#L41

So, I added a missing gem dependency to gemspec.