按照官网上的第二种方式安装：

>bundle install   

Updating http://github.com/hh/em-winrm.git   
Fetching gem metadata from http://rubygems.org/.........    
Fetching gem metadata from http://rubygems.org/..    
Resolving dependencies...   
Using rake (10.0.3)    

NoMethodError: undefined method size for nil:NilClass    
An error occurred while installing libxml-ruby (2.6.0), and Bundler cannot continue.                                   
Make sure that gem install libxml-ruby -v '2.6.0' succeeds before bundling.   

>gem install libxml-ruby -v '2.6.0'    

ERROR: While executing gem ... (NoMethodError)    
undefined method 'size' for nil:NilClass