Gracefully handle Ruby < 1.9 in interrupt catching spec

Author: myrridin

spec/rspec/core/runner_spec.rb

@@ -115,7 +115,7 @@ module RSpec::Core
       let(:interrupt_handlers) { [] }
 
       before do
-        if RUBY_ENGINE == "jruby"
+        if const_defined?(RUBY_ENGINE) && RUBY_ENGINE == "jruby"
           allow(Signal).to receive(:trap).with("INT", nil) do |&block|
             interrupt_handlers << block
           end