CI: Travis Slave Crashes on Master + PRs

[gfyoung]

https://travis-ci.org/pandas-dev/pandas/jobs/282616908

Looks like a pytables issue of some sort but not sure.

[jreback]

going to leave this issue open, so we can revert #17760 once conda-forge builds are good again.

[TomAugspurger]

Moving this off the 0.21 milestone, since it's not a blocker.

[jreback]

yeah its fine, we are pinned in any event.