stop drb service in example

Fixes intermittent order-dependent failure:

Before:

  bin/rspec --seed 15528
  ...
  1151 examples, 1 failure, 1 pending

  Failed examples:
  rspec ./spec/rspec/core/configuration_spec.rb:393

After:

  bin/rspec --seed 15528
  ...
  1151 examples, 0 failures, 1 pending