I had the same problem. After digging, I found the solution. The problem is most likely to be with the version of RubyGems installed in your system. (It would be <1.8.25)
Till that version, RubyGems didn't roundtrip gemspecs properly and thus couldn't parse "< 2.1, >= 1.8" correctly.
So, go through these steps:
gem uninstall ZenTest
gem update --system
gem install ZenTest
The problem should get solved and you should no longer receive the warnings.