BUG: IntervalArray.astype(categorical_dtype) losing ordered
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

looks fine, can you add a whatsnew note to document, ping on green. whatsnew added, greenish (same test_area_lim travis failure) ping