Fixup Rakefile deprecations

If RSpec is already loaded (it's common to do:
```
require 'rspec' # or 'rspec/core'
require 'rspec/core/rake_task'
```
in a `Rakefile`), and we can safely use the built in deprecation support however otherwise it's become to complicated to load the deprecation file alone so we drop back to a simplier warn.

If we don't do this, the deprecated methods will blow up unless `require 'rspec/core'` has been required, or we have to require most of `rspec/core` due to the use of `configuration` and the formatters.