Remove overabundant parenthesis

Addressing:
/usr/sbin/foreman-rake db:migrate
rake aborted!
/opt/rh/ruby193/root/usr/share/gems/gems/scaptimony-0.0.1/db/migrate/20141015115511_add_arf_report_unique_constraint.rb:3:
syntax error, unexpected ')', expecting keyword_end
Force own index name

Addressing:
Index name 'index_scaptimony_arf_reports_on_assert_id_and_policy_id_and_date_and_digest' on table 'scaptimony_arf_reports' is too long; the limit is 63 characters
Correct the column name

Addressing:
PGError: ERROR:  column "assert_id" does not exist : CREATE UNIQUE INDEX "index_scaptimony_arf_reports_unique_set" ON "scaptimony_arf_reports" ("assert_id", "policy_id", "date", "digest")
Take Asset instance from the caller.