how can i install in this lacal -n /usr/local/bin cocoapods 

```
Successfully installed concurrent-ruby-1.2.2
Successfully installed cocoapods-core-1.12.0
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /usr/bin directory.
```
    
because i don't have write permissions for the /usr/bin directory,so how can i  install in this lacal -n /usr/local/bin cocoapods 
