我正在尝试使用rvm更新ruby。当我使用以下命令时，我分别得到以下错误。请任何人帮我搭车。
[prabhu@10.100.202.8 ~]$ rvm install ruby-1.9.3-p547
Searching for binary rubies, this might take some time.
No binary rubies available for: osx/10.8/x86_64/ruby-1.9.3-p547.
Continuing with compilation. Please read 'rvm help mount' to get more information on binary rubies.
Checking requirements for osx.
/usr/local/Library/Homebrew/global.rb:57: warning: Insecure world writable dir /usr in PATH, mode 040777
ERROR: 'Error: Unknown command: --cellar/bin' is not writable - it is required for Homebrew, try 'brew doctor' to fix it!
Requirements installation failed with status: 1.