Update ZenTesk to fix installation with RubyGems 2.0
Update almost all gems

Note that the notifo notifier have been removed since the service do not exist
anymore and it was pulling a lot of outdated gems
Update rubygems in travis
Target ZenTest 4.8.4 to be compatible with both rubygems < 1.8 and >= 2
Upgrade bundler to 1.3.1 before runing CI to workaround the rubygem/bundler/ZenTest mess