Use SchemaTool::updateSchema instead of createSchema

[pascalheidmann-bedarf]

The generator uses the SchemaTool::createSchema call in the generated bootstrap if models are added.
As the function is called if you reinstall the plugin it tries to create the model's database tables but fails.
Therefore calling SchemaTool::updateSchema($classes, true); instead is preferable as it also creates the tables if they aren't present but won't raise any errors.