Gemfile sources https://rubygems.org not http://ruby.taobao.org

This alternate source crept in e0a39481180cda072ba2c0e64b275661e96d61f3, presumably by accident.

This is currently causing Travis build to fail, as the alternate gem source doesn't have ZenTest-4.9.0 available.

It's also a terrible security risk, especially in light of the RubyGems.org compromise earlier this year.