ZenTest 0.4.9 can't be installed with older versions of rubygems.
It's a common and documented practice on Travis: http://about.travis-ci.org/docs/user/languages/ruby/#Upgrading-RubyGems

The issue is that as always the rubygems team have pushed some bugs. So rubygems 2.0.2 do not work with bundler anymore I try to find a workaround