do not use ZenTest 4.8.4

addressing:
324RUNNING: RAILS_ENV=development bundle exec compass compile

325Invalid gemspec in [/home/travis/.rvm/gems/ruby-1.8.7-p371/specifications/ZenTest-4.8.4.gemspec]: Illformed requirement ["< 2.1, >= 1.8"]

326sh: 1: rpm: not found

327Invalid gemspec in [/home/travis/.rvm/gems/ruby-1.8.7-p371/specifications/ZenTest-4.8.4.gemspec]: Illformed requirement ["< 2.1, >= 1.8"]

328Invalid gemspec in [/home/travis/.rvm/gems/ruby-1.8.7-p371/specifications/ZenTest-4.8.4.gemspec]: Illformed requirement ["< 2.1, >= 1.8"]

329sh: 1: rpm: not found

330Invalid gemspec in [/home/travis/.rvm/gems/ruby-1.8.7-p371/specifications/ZenTest-4.8.4.gemspec]: Illformed requirement ["< 2.1, >= 1.8"]

Workaround for
https://github.com/seattlerb/zentest/issues/28