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.

In https://github.com/rspec/rspec-expectations/blob/master/features/built_in_matchers/equality.feature there is some discussion about how `should be(b)` and `should eq(b)` are good substitutes if you want to avoid the warning for `should ==`, I left that in but changed all the examples to use `expect`. Not sure if that still makes any sense.

I also changed the examples in the equality file that were talking specifically about `.should ==` to use the awkward `.to be ==`. Not sure if we want to keep that around, since `.to be` ends up being the same thing.