rspec/rspec-core — pull request #1118 by myronmarston (2013-10-20 22:55:33+00:00)
Title: Fail build if warnings are emitted.

This is another go at what @xaviershay was trying to achieve in #1117. I tried it by introducing the warning I mentioned in the comments there, and it works :).

Basically, this relies on the fact that ruby prints its warnings to $stderr.