Change RSpec/MultipleExpectations cop to AggregateFailuresByDefault
Change RSpec/MultipleExpectations cop to AggregateFailuresByDefault

This setting requires carefully judgement.
But I decided to set `meta[:aggregate_failures]` in spec_helper to implove performance.

"1 example has 1 assertion" is good practice.
Let's keep this with code review.

backus/rubocop-rspec pull/511

onk/onkcop,onk/onkcop