Seems this failing due to addressable but that requires rack-mount but only used in development. Going off a suggestion from a coworker as this seems as an odd dependency to have but most likely should just be set for dev.
Rack isn't used at all so no need to specify. Based on [this comment](https://github.com/bitpay/ruby-client/pull/51#issuecomment-236736392) this seems to be carried over from a Rails app so is safe to remove. This will also allow this gem to be used in Rails 5 apps which now require Rack v2.
No need to specify json dependency since this gem requires Ruby 2.0+ and I've [seen this change made elsewhere](https://github.com/mperham/sidekiq/issues/2743) to clean up dependencies and also support other platforms (Windows). The json library is being required in `lib/bitpay/client.rb`.
