TST: Conda CI build not using latest wheels?

[tswast]

See: #247 (comment)

The Conda build is likely using an old version of pandas, even though it's supposed to be using pre-release wheels.

[tswast]

I do see pandas 0.25 nightly builds on https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com/ so it's not as simple as changing the URL. Next steps to debug should probably be to print out the version of pandas in the https://github.com/pydata/pandas-gbq/blob/master/ci/run_conda.sh script.

Actually, I think I figured it out.

https://github.com/pydata/pandas-gbq/blob/8a263453e66b29dcfaed018bd0714d669bfa44e2/.circleci/config.yml#L53

should set PANDAS=MASTER if we want to use the nightlies on the conda build. Then again, maybe it's still useful to have some minimum version of pandas build. So if that's currently the oldest supported, we should make sure one of the pip builds has that pandas version.