Change the prefix of SolidusBraintree tables from solidus_paypal_braintree to solidus_braintree

[gsmendoza]

Background

This was pushed back from #99 because we didn't want the table rename to block users from migrating to v2.0.0 of SolidusBraintree. We were also not sure whether to force users to rename to the new prefix or continue supporting the old prefix for existing users.

There was also a concern about the impact of a table rename for large databases and whether a scalable approach (e.g. duplicating the table, double writing, phasing out the old one) is the responsibility of the extension or we can proceed with a vanilla rename_table and let anything more complex than that to be handled by each store.