Fix failing MySQL spec due to deadlock condition
spec/features/uploads/user_uploads_file_to_note_spec.rb was failing in
master because MySQL detected a deadlock when a DELETE and INSERT for
the same indexed item occurred within a transaction in the uploads
table. Due to InnoDB's next-key locking algorithm
(see innodb_locks_unsafe_for_binlog in https://dev.mysql.com/doc/refman/5.5/en/innodb-parameters.html), InnoDB
sets an exclusive lock for any of the indexed records it encounters,
so the INSERT will fail until the DELETE is committed.
To fix this, we just disable the transaction for MySQL and keep it for PostgreSQL.
Edited by Stan Hu