When you encounter

```
/Users/kernel/.gem/ruby/2.0.0/gems/mysql2-0.3.16/lib/mysql2.rb:8:in 
  `require': dlopen(/Users/kernel/.gem/ruby/2.0.0/gems/mysql2-0.3.16/lib/mysql2/mysql2.bundle, 9): 
    Library not loaded: 
      /usr/local/lib/libmysqlclient.18.dylib (LoadError)
```

Try `gem pristine mysql2`, this command restores installed gems to pristine condition from files located in the gem cache