BLD: Set max version of numba to 0.48.0
This should fix the errors in the CI (Travis) builds, e.g.

```
E       AttributeError: module 'numba' has no attribute 'targets'
```
makes sense, cc @mroeschke  Ah. This had slipped my mind.

The reason this is failing is due to https://github.com/numba/numba/issues/5256#issuecomment-586899566

I'm working on the patch right now that would be preferable to pinning the numba version, but if this is blocking anything urgent, happy to merge https://github.com/pandas-dev/pandas/pull/33687 should equivalently fix the failing builds due to numba v0.50.0 Sounds good, closing in favor of #33687  @mgmarino thanks for the catch. Merged in https://github.com/pandas-dev/pandas/pull/33687 so things should be working now