Refactor jump_to and its spec

Update jump_to command's spec to RSpec 3 (https://github.com/pry/pry/issues/1294)
Perform some refactorings in lib along the way.
RSpec is moving away from monkey patching Object. We see this in `expect(obj).to ...` vs `obj.should ....`

In this case, they want to move away from placing `describe` on Object, as well. So they place it on RSpec directly. [here](https://github.com/rspec/rspec-core/commit/de01e0567f5ca49ad445d90387e947ad116f2081) and [here](https://github.com/rspec/rspec-core/commit/ec340c473d147b360bebd354fcaf8fed4ceb7b93).