Upgrade nano on Mac OS X
When running a binary from your shell, the shell stores its path within PATH in a cache and uses that value for future calls to avoid having to research the path each time. If you then install a new version of this binary in a different location the shell won't notice this and keep running the previous version.
If you are running bash

hash -t nano will show the currently used version of nano (if any)
hash nano will update the cache for nano

If you are running zsh

hash -m nano will show the currently used version of nano (if any)
hash -f nano will update the cache for nano