Update cukes to new syntax

Part of https://github.com/rspec/rspec-core/issues/1055

Pretty much everything in the "built_in_matchers" directory uses `it { should }` style assertions, which have no corresponding `expect` equivalent, so I just left those alone.

I opened #333 because I couldn't change that spec, seems the new `expect().to match(regexp)` equivalent doesn't do the same diffing upon failure.