Add fatal error when mysql 5.6 is used with ubuntu 12.04

Fixes #63

- When specifying 5.6 for Ubuntu 12.04, the upstream mysql cookbook will break when trying to install mysql-server. See these lines: https://github.com/opscode-cookbooks/mysql/blob/master/libraries%2Fhelpers.rb#L156-L158
Have added a fatal error to be more verbose and specific about the error, as the standard error isn't very helpful. This can be removed once upstream supports it, if it ever does.
- Removed the include_recipe mysql::client since database::mysql includes it already.

stack_commons testing so far will pass on this, but we need to write tests in anything that includes stack_commons where there's a requirement to use 12.04 and mysql 5.6 so that it doens't fail.
Just leaving a note here in case someone can use it. The `mysql-chef_gem` cookbook actually adds `mysql::client` back to the runlist -- https://github.com/opscode-cookbooks/mysql-chef_gem/blob/master/libraries/provider_mysql_chef_gem.rb#L16-L18. So this was going to happen anyway.