ZenTest "invalid gemspec" / "Illformed requirement"

Today I ran into this:

Invalid gemspec in [/usr/local/rvm/gems/ruby-1.9.3-p194/specifications/ZenTest-4.9.3.gemspec]: Illformed requirement ["< 2.1, >= 1.8"].

You need a newer Rubygems version. Try this: gem update --system 1.8.29