Show warning for RSpec examples that has no expectation

You may sometimes forget to write should for matchers.

it { a == b } # ng
it { a.should == b } # ok

Using the above RSpec configuration, that warns for such cases.

$ rspec -fd spec/a_spec.rb
[WARN] No expectation in example at ./spec/a_spec.rb:4: You may forget to write `should` in the example
  example at ./spec/a_spec.rb:4

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