gem安装时出现 undefined method `size' for nil:NilClass (NoMethodError) 的解决办法

终端输入gem env

得到gem的PATH路径，比如

- GEM PATHS:
- /usr/local/ruby/lib/ruby/gems/2.1.0
- /home/vagrant/.gem/ruby/2.1.0

那么分别cd  /usr/local/ruby/lib/ruby/gems/2.1.0
　　　　cd  /home/vagrant/.gem/ruby/2.1.0

分别输入rm -rf cache