Set correct packagename for ruby_mysql on Ubuntu 18.04

Author: osmith

manifests/params.pp

@@ -223,6 +223,7 @@
         'stretch'          => 'ruby-mysql2',
         'trusty'           => 'ruby-mysql',
         'xenial'           => 'ruby-mysql',
+        'bionic'           => 'ruby-mysql2',
         default            => 'libmysql-ruby',
       }
     }