The NewRelic gem provides simple RSS usage implementations for a number of operating systems and ruby runtimes with their MemorySampler class.
Include the newrelic_rpm gem in your Gemfile and invoke it thus:
NewRelic::Agent::Samplers::MemorySampler.new.sampler.get_sample
and it returns the number of megabytes of memory the current process holds as the RSS.
The implementation prefers in-process counters where available (jruby), use the /proc/#{$$}/status on Linux, and fall back to ps everywhere else.