Or little computing tricks and hacks
After recently upgrading to Ubuntu 15.04 (almost ready for the next release!), my gem dependencies were broken.
... /lib/mysql2.rb:31:in `require': Incorrect MySQL client library version! This gem was compiled for 5.5.43 but the client library is 5.6.24. (RuntimeError)
bundle update changed the gem. Only uninstalling the gem forced the correct update:
gem uninstall mysql2 bundle install
Solution taken from http://stackoverflow.com/questions/10725439/incorrect-mysql-client-library-version.