remove developer preview warning from .combined_status()

The combined_status API endpoint is no longer in development preview. This patch makes the following changes:

* removes the HTTP `Accept` header that was required for this call while it was in its preview period
* remove the warning printed about the method being in the development preview

All tests passed locally when following the steps in the contribution information. Please let me know if there are any changes that are needed to be made.
Merge pull request #505 from theckman/remove_preview_warning_from_combined_status

remove developer preview warning from .combined_status()