CI: Pin NumPy version
closes #18992 

This should fail.
hmm, there *is* a compat version of numba that should be ok on 3.5/1.10. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18993?src=pr&el=h1) Report
> Merging [#18993](https://codecov.io/gh/pandas-dev/pandas/pull/18993?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9c1071be085153d2b7d06417f5b2056ed86a8a70?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18993/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18993?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18993      +/-   ##
==========================================
- Coverage   91.58%   91.56%   -0.03%     
==========================================
  Files         150      150              
  Lines       48972    48972              
==========================================
- Hits        44851    44839      -12     
- Misses       4121     4133      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.92% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18993?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18993/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18993?src=pr&el=footer). Last update [9c1071b...620216d](https://codecov.io/gh/pandas-dev/pandas/pull/18993?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 so #18994 actually makes this error show up (and that is why I think you have it on 0.22.x branch, its using the latest conda), while on master I pinned to 4.3.30 which I thought was causing the issues. https://travis-ci.org/pandas-dev/pandas/jobs/322965133.

 just going to skip for old numpy for now on those tests. i don't think its a big deal.

you can cherry-pick the commit to 0.22.x closed in favor of #18995 