Add mysql gem version string for activerecord

Not to install gem newer than 2.8.

mysql gem 2.9.0 has been out, so project `bundle` will install mysql 2.9.0.

But ActiveRecord 3.2.x mysql adapter requires mysql gem '~> 2.8'

* https://github.com/rails/rails/blob/3-2-stable/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

so a Gemfile created by `padrino g project -a mysql` occurs a confusing error... "stack level too deep"

This fix should be reverted when Rails 4 released

* https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

Thanks!