Add pandas package to appveyor configuration

[fkohlgrueber]

PR Summary

I appended pandas to the list of packages that are installed into the conda environment that's used for testing on Appveyor.

This lets all tests that depend on pandas run (currently 7). Two of the Travis CI jobs already include pandas and I think that there's no reason not to run these tests on Appveyor as well. Please tell me if there are any.

[tacaswell]

Thanks for contribution!

My only concern is if the pandas tests add significant overhead (as appevyor is already very slow). 👍 if this adds < 30s to the run time.

[QuLogic]

Comparing with a random build from master, this PR is 3 minutes slower, but comparing with a random build from a different PR, this PR is 12 minutes faster. So I think whatever the difference is, it's well within the variance of AppVeyor build times. I'll take @tacaswell's conditional 👍 as granted, then.