raise exceptions on `expect(..) == ..` syntax

I caught myself replacing old `x.should == y` syntax with `expect(x) == y`. As you might imagine, that does absolutely nothing. Half of my specs were not testing code, some were broken.

I fear that this might happen to others. This raises an exception to a common `==` matcher.
Rubocop would like this to be prefixed with an underscore as it's not used.