BLD: fix appveyor to work with latest conda/conda-build

https://ci.appveyor.com/project/jreback/pandas-465/build/job/ic8ytmkwcc8nfucc

Current are
- `conda==4.1.1` is good
- `conda-build==1.21.0` seems buggy

a working version is:
- `conda=4.0.8`
- `conda-build=1.20.3`

may need to fix the version if conda-build not fixed soon
