[5.6] Prevent calling the bootable trait boot method twice

[themsaid]

The User model in Spark for example uses the Spark\Billable trait which uses the Cashier\Billable trait, in Model::bootTraits method class_uses_recursive will return the two different Billable traits in the results, so if the Billable trait has a bootBillable method it'll be called twice:

One when model::bootTrairs found Spark\Billable and one when it found Cashier\Billable.

This PR prevents calling the trait boot method multiple times by keeping track of what methods were called and prevent calling them again.