using unique bucket for multi concurrency testing

[why404]

确保单元测试里边用到的测试 Bucket 全局唯一

使得10多种不通的 Ruby 宿主环境能隔离互不影响地执行单元/集成测试

Ruby 宿主环境如下

1.8.7, 1.9.2, 1.9.3, jruby-18mode, jruby-19mode, rbx-18mode, rbx-19mode, ruby-head, jruby-head, ree

详见 https://travis-ci.org/qiniu/ruby-sdk

Finished in 1 minute 56.17 seconds 60 examples, 0 failures

该调整已通过测试，可以直接 merge