Ran across this today, to solve it I did:
brew rm -f readline
brew install readline
brew link readline --force
Hope it helps.
EDIT: I recently ran into this problem again (after downgrading Ruby) since I wrote this, and I now prefer @califrench’s solution from the comments below:
ln -s /usr/local/opt/readline/lib/libreadline.dylib /usr/local/opt/readline/lib/libreadline.7.dylib