gem mysql2 installation error on Mac

Adler @ 2018-06-20

mysql2 version 0.3.20 could be a problem when we have multiple versions of mysql installed. So the solution is to link only the old version but not the latest version (8.0.11 at the moment).

brew unlink mysql
brew link mysql@5.6 --force
gem install mysql2 --version 0.3.20