Fix "allowed failures" Travis-CI builds for pydap and dev versions of netCDF4 and pandas

[shoyer]

These are useful for catching changes upstream before they turn into bugs for xarray users, but they are currently failing for spurious reasons.

If someone has time to investigate and fix these, that would be great!

[stale]

In order to maintain a list of currently relevant issues, we mark issues as stale after a period of inactivity
If this issue remains relevant, please comment here; otherwise it will be marked as closed automatically

[jhamman]

I've just opened #2736 to close this issue. In actuality, this issue could be closed as is because:

1. The pydap and netcdf4 builds were removed in Remove broken Travis-CI builds #2661
2. The pandas dev build is passing in recent travis builds