修正 ruby 2.0.0 运行 zeus server 时的错误

````bash
~/.rvm/gems/ruby-2.0.0-p0@global/gems/bundler-1.3.0.pre.8/lib/bundler/runtime.rb:33:in `block in setup': 
You have already activated json 1.7.7, but your Gemfile requires json 1.7.6. 
Using bundle exec may solve this. (Gem::LoadError)
```