I ran into this problem after a complete removal and then fresh install of MySQL. Specifically:
Library not loaded: /usr/local/opt/mysql/lib/libmysqlclient.20.dylib
I had not even touched my Rails app.
Reinstalling the mysql2
gem solved this problem.
$ gem uninstall mysql2
$ gem install mysql2 -v 0.3.18 # (specifying the version found in my Gemfile.lock)
[MySQL 5.7.10, Rails 4.0.0, Ruby 2.0.0, Mac OS X Yosemite 10.10]