[SPEC] Make spec a bit more lax for ruby-2.5

Author: sfgeorge

spec/sippy_cup/runner_spec.rb

@@ -534,7 +534,7 @@ def active_thread_count
         end
 
         it "proxies stderr to the terminal" do
-          expect(capture(:stderr) { subject.run }.strip).to eq(error_string)
+          expect(capture(:stderr) { subject.run }.strip).to include(error_string)
         end
 
         it "does not leak threads" do