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`), 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.
Merge pull request #984 from rspec/fixup_rake_deprecate

Fixup Rakefile deprecations