Modified postgres adapter to support insert ignore policy m...

...achine assignments

569 examples, 0 failures
Coverage report generated for RSpec to /Users/yzhang/the_policy_machine/coverage. 629 / 707 LOC (88.97%) covered.
Passes with postgres and mysql adapters and in Dalton
I'd leave this as `to_not raise_error` with no argument, since if it raises some error other than `ActiveRecord::RecordNotUnique` the spec should still probably fail.