Looks like ost-sdk-ruby uses the rack gem, but does not specify it in the gemspec.
This issue was resolved by @Jason . There was a missing step in the documentation to install rack.
The solution is to issue the following command
sudo gem install rack
PS: to use the gem ‘rack’ - ruby version 2.2.x is required
Well, that's not a missing step. That's a workaround. Solution is for ost-sdk-ruby to specify the dependency in the gemspec.