fix issue #42: nokogiri 1.7.0 does not support ruby 2.0 or earlier
fix issue #42: nokogiri 1.7.0 does not support ruby 2.0 or earlier

Change-Id: Ia9e12a5ce2476ffe85cda43bda238385bbd0af8e

aliyun/aliyun-oss-ruby-sdk,luluhongxu/aliyun-oss-ruby-sdk,luluhongxu/aliyun-oss-ruby-sdk,aliyun/aliyun-oss-ruby-sdk