Remove unneeded requires in rake task.

- rspec/core/version require was added in
  390392e7e76429bbeddedd615565e858c70194f9 but
  looks like it was never actually used.
- We used to have some warnings emitted by the rake
  task (and thus needed to load that support) but
  don’t have any more warnings.

@postmodern, do you remember why you added `require 'rspec/core/version'` in that commit?