Mac 下使用brew install 报错：
localhost:infer-osx-v0.6.0 admin$ sudo brew install opam
Error: Cowardly refusing to `sudo brew install'
You can use brew with sudo, but only if the brew executable is owned by root.
However, this is both not recommended and completely unsupported so do so at
your own risk.
用sudo执行brew命令，必须将 /usr/local/bin/brew 改成root
sudo chown root:wheel /usr/local/bin/brew
出现 warning: Insecure world writable dir /usr/local/bin in PATH, mode 040777
sudo chmod go-w /usr/local/bin